Use unittest.mock instead of third party mock

Now that we don't need to support py27, we can use the standard library
unittest.mock module instead of the third party mock lib.

Change-Id: I67b86898cb443dc02c6b41a78bfe54f43f8d6c74
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

oslo_upgradecheck/tests/test_upgradecheck.py

@@ -22,8 +22,8 @@ Tests for `upgradecheck` module.
 import os.path
 import subprocess
 import sys
+from unittest import mock
 
-import mock
 from oslo_config import cfg
 from oslotest import base