migrate from external mock package to stdlib unittest.mock

It was moved to the stdlib in python 3.3, and the PyPI version is a
backport for python versions less than 3.3.

tests/tests.py

@@ -2,7 +2,7 @@
 import re
 import platform
 import unittest
-import mock
+from unittest import mock
 
 import django
 from django.db import connection