Enable BytesWarning during test and fix discovered case (#1322)

The Python command line argument -b causes Python to emit a warning when
bytes and str usage is mixed. This is generally considered bad practice
as either one or the other is required. Enabling this feature during
tests helps catch them before reaching production.

The warning appeared as:

    tests/test_scripting.py::TestScripting::test_eval_msgpack_pipeline_error_in_lua
      .../redis-py/redis/client.py:3967: BytesWarning: str() on a bytes instance
        cmd = ' '.join(imap(safe_unicode, command))

redis/_compat.py

@@ -166,14 +166,18 @@ def itervalues(x):
     def nativestr(x):
         return x if isinstance(x, str) else x.decode('utf-8', 'replace')
 
+    def safe_unicode(value):
+        if isinstance(value, bytes):
+            value = value.decode('utf-8', 'replace')
+        return str(value)
+
     next = next
     unichr = chr
     imap = map
     izip = zip
     xrange = range
     basestring = str
     unicode = str
-    safe_unicode = str
     long = int
     BlockingIOError = BlockingIOError
 

tox.ini

@@ -9,7 +9,7 @@ deps =
     pytest >= 2.7.0
 extras =
     hiredis: hiredis
-commands = {envpython} -m coverage run -m pytest -W always {posargs}
+commands = {envpython} -b -m coverage run -m pytest -W always {posargs}
 
 [testenv:pycodestyle]
 basepython = python3.6