Use unittest.mock instead of third party mock

Now that we no longer support py27, we can use the standard library
unittest.mock module instead of the third party mock lib.

Change-Id: I017716842c61b814bbe16cc2b8788f160f4ad9cd
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>
diff --git a/patrole_tempest_plugin/tests/unit/fixtures.py b/patrole_tempest_plugin/tests/unit/fixtures.py
index 41afe9b..2ce1d08 100644
--- a/patrole_tempest_plugin/tests/unit/fixtures.py
+++ b/patrole_tempest_plugin/tests/unit/fixtures.py
@@ -15,9 +15,9 @@
 
 """Fixtures for Patrole tests."""
 from __future__ import absolute_import
+from unittest import mock
 
 import fixtures
-import mock
 import time
 
 from tempest.common import credentials_factory as credentials