Use unittest.mock if available

Use unittest.mock https://docs.python.org/3/library/unitetst.mock.html
starting from >= Python 3.3

flake8/tests/test_engine.py

@@ -1,7 +1,10 @@
 from __future__ import with_statement
 
 import unittest
-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock  # < PY33
 
 from flake8 import engine, util, __version__