Remove unnecessary __future__ imports (#6096)

Removes `from __future__ import generators, nested_scopes, with_statement` from every header + updates header checker.

These future imports are [enabled by default since Python 2.6]( https://docs.python.org/2.7/reference/simple_stmts.html#future-statements) so they were redundant. 

Removing them reduces duplication and simplifies code base ever so slightly.

We'll also be doing this type of change several times once we drop Python 2, e.g. removing `__future__` entirely and removing `from builtins` imports, so this is a good first step.

#### FYI commands to do this 
* `grep -rli 'from\ __future__\ import' src tests pants-plugins examples contrib | xargs perl -pi -e 's/.*from __future__ import.*/from __future__ import absolute_import, division, print_function, unicode_literals/'`
* `grep -rli 'from\ __future__\ import' src tests pants-plugins examples contrib | xargs perl -ni -e 'print unless /.*with_statement.*/'`

## Impact on PRs
Pre-existing PRs either must be merged first or rebased onto this after it's merged.

build-support/bin/check_header_helper.py

@@ -8,8 +8,8 @@
 #
 # usage: check_header_helper.py dir1 [ dir2 [ ... ] ]
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
+
 import os
 import re
 import sys
@@ -18,8 +18,7 @@
 # Copyright YYYY Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 """
 
@@ -29,7 +28,7 @@ def check_header(filename):
   try:
     with open(filename, 'r') as pyfile:
       buf = ""
-      for lineno in range(1,8):
+      for lineno in range(1,7):
         line = pyfile.readline()
         # Skip shebang line
         if lineno == 1 and line.startswith('#!'):
@@ -45,6 +44,7 @@ def check_header(filename):
   except IOError:
     return False
 
+
 def check_dir(directory):
   """Returns list of files that fail the check."""
   failed_files = []
@@ -71,5 +71,6 @@ def main():
     print('  {}'.format('\n  '.join(failed_files)))
     sys.exit(1)
 
+
 if __name__ == '__main__':
   main()

contrib/avro/src/python/pants/contrib/avro/register.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from pants.build_graph.build_file_aliases import BuildFileAliases
 from pants.goal.task_registrar import TaskRegistrar as task

contrib/avro/src/python/pants/contrib/avro/targets/java_avro_library.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from pants.backend.jvm.targets.jvm_target import JvmTarget
 from pants.base.payload import Payload

contrib/avro/src/python/pants/contrib/avro/tasks/avro_gen.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import os
 

contrib/avro/tests/python/pants_test/contrib/avro/tasks/test_avro_gen.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from textwrap import dedent
 

contrib/avro/tests/python/pants_test/contrib/avro/tasks/test_avro_gen_integration.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import os
 

contrib/buildgen/src/python/pants/contrib/buildgen/build_file_manipulator.py

@@ -2,8 +2,7 @@
 # Copyright 2014 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import ast
 import logging

contrib/buildgen/tests/python/pants_test/contrib/buildgen/test_build_file_manipulator.py

@@ -2,8 +2,7 @@
 # Copyright 2014 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from textwrap import dedent
 

contrib/buildrefactor/src/python/pants/contrib/buildrefactor/buildozer.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import logging
 

contrib/buildrefactor/src/python/pants/contrib/buildrefactor/buildozer_binary.py

@@ -2,8 +2,7 @@
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import logging
 

contrib/buildrefactor/src/python/pants/contrib/buildrefactor/meta_rename.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import logging
 from collections import defaultdict

contrib/buildrefactor/src/python/pants/contrib/buildrefactor/register.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from pants.goal.task_registrar import TaskRegistrar as task
 

contrib/buildrefactor/tests/python/pants_test/contrib/buildrefactor/buildozer_util.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 
 def prepare_dependencies(self):

contrib/buildrefactor/tests/python/pants_test/contrib/buildrefactor/test_buildozer.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import os
 import re

contrib/buildrefactor/tests/python/pants_test/contrib/buildrefactor/test_buildozer_integration.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from pants_test.pants_run_integration_test import PantsRunIntegrationTest
 

contrib/buildrefactor/tests/python/pants_test/contrib/buildrefactor/test_meta_rename.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import os
 

contrib/buildrefactor/tests/python/pants_test/contrib/buildrefactor/test_meta_rename_integration.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from pants_test.pants_run_integration_test import PantsRunIntegrationTest
 

contrib/codeanalysis/src/python/pants/contrib/codeanalysis/register.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from pants.goal.task_registrar import TaskRegistrar as task
 

contrib/codeanalysis/src/python/pants/contrib/codeanalysis/tasks/bundle_entries.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import os
 import shutil

contrib/codeanalysis/src/python/pants/contrib/codeanalysis/tasks/extract_java.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import os
 

contrib/codeanalysis/src/python/pants/contrib/codeanalysis/tasks/index_java.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import os
 

contrib/codeanalysis/src/python/pants/contrib/codeanalysis/tasks/indexable_java_targets.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from pants.backend.jvm.targets.jvm_target import JvmTarget
 from pants.build_graph.target_scopes import Scope

contrib/codeanalysis/tests/python/pants_test/contrib/codeanalysis/tasks/test_index_java_integration.py

@@ -2,8 +2,7 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import glob
 import os

contrib/confluence/src/python/pants/contrib/confluence/register.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from pants.goal.task_registrar import TaskRegistrar as task
 

contrib/confluence/src/python/pants/contrib/confluence/tasks/confluence_publish.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import os
 import textwrap

contrib/confluence/src/python/pants/contrib/confluence/util/confluence_util.py

@@ -2,8 +2,7 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import getpass
 import logging

contrib/cpp/src/python/pants/contrib/cpp/register.py

@@ -2,8 +2,7 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from pants.build_graph.build_file_aliases import BuildFileAliases
 from pants.goal.task_registrar import TaskRegistrar as task

contrib/cpp/src/python/pants/contrib/cpp/targets/cpp_binary.py

@@ -2,8 +2,7 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from pants.base.payload import Payload
 from pants.base.payload_field import PrimitiveField

contrib/cpp/src/python/pants/contrib/cpp/targets/cpp_library.py

@@ -2,8 +2,7 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from pants.contrib.cpp.targets.cpp_target import CppTarget
 

contrib/cpp/src/python/pants/contrib/cpp/targets/cpp_target.py

@@ -2,8 +2,7 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from pants.base.payload import Payload
 from pants.build_graph.target import Target

contrib/cpp/src/python/pants/contrib/cpp/tasks/cpp_binary_create.py

@@ -2,8 +2,7 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import os
 

contrib/cpp/src/python/pants/contrib/cpp/tasks/cpp_compile.py

@@ -2,8 +2,7 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import os
 

contrib/cpp/src/python/pants/contrib/cpp/tasks/cpp_library_create.py

@@ -2,8 +2,7 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import os
 

contrib/cpp/src/python/pants/contrib/cpp/tasks/cpp_run.py

@@ -2,8 +2,7 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from pants.base.workunit import WorkUnitLabel
 

contrib/cpp/src/python/pants/contrib/cpp/tasks/cpp_task.py

@@ -2,8 +2,7 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 from pants.base.exceptions import TaskError
 from pants.task.task import Task

contrib/cpp/src/python/pants/contrib/cpp/toolchain/cpp_toolchain.py

@@ -2,8 +2,7 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import os