Fix pyflakes and pep8 errors

Use extra asserts to use previously unused variables in tests,
such as `cass` and `response`.

Fix only pyflakes errors in docs/conf.py

tests/integration/test_register_serializer.py

@@ -10,7 +10,7 @@ class MockSerializer(object):
     def deserialize(self, cassette_string):
         self.serialize_count += 1
         self.cassette_string = cassette_string
-        return {'interactions':[]}
+        return {'interactions': []}
 
     def serialize(self, cassette_dict):
         self.deserialize_count += 1