Drop use of third party mock

The third party mock library was needed for mocking support across py2
and py3. Now that we have dropped py2 support, we can use the standard
library unittest.mock instead.

Change-Id: I8b0db843743ba234f35f54e29ca820c909102059
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

cloudkittydashboard/tests/test_predictive_pricing.py

@@ -13,9 +13,9 @@
 # under the License.
 import json
 import os
+from unittest import mock
 
 from django import http
-import mock
 
 from cloudkittydashboard.tests import base