Handle missing 'tests_require' option in newer setuptools (#59)

This option was outright removed from newer versions of setuptools and
is no longer visible at all. The existing behavior always raises an
exception even if the unsupported option isn't used in the package.

Author: cottsay

colcon_python_setup_py/package_augmentation/python_setup_py.py

@@ -44,7 +44,7 @@ def augment_package(  # noqa: D102
         ]:
             desc.dependencies[dependency_type] = {
                 create_dependency_descriptor(d)
-                for d in config[option_name] or ()}
+                for d in config.get(option_name) or ()}
 
         def getter(env):
             nonlocal setup_py