Use unittest.mock instead of mock

The mock third party library was needed for mock support in py2
runtimes. Since we now only support py36 and later, we can use the
standard lib unittest.mock module instead.

Change-Id: If7d749018d7a69906f7f4763b147f09cfadd6692

refstack_client/tests/unit/test_client.py

@@ -21,8 +21,8 @@ import os
 import tempfile
 
 import httmock
-import mock
-from mock import MagicMock
+from unittest import mock
+from unittest.mock import MagicMock
 import unittest
 
 from refstack_client import refstack_client as rc

refstack_client/tests/unit/test_list_parser.py

@@ -20,7 +20,7 @@ import subprocess
 import tempfile
 
 import httmock
-import mock
+from unittest import mock
 import unittest
 
 import refstack_client.list_parser as parser

test-requirements.txt

@@ -4,6 +4,5 @@ flake8<3.8.0
 docutils>=0.11  # OSI-Approved Open Source, Public Domain
 stestr>=1.1.0 # Apache-2.0
 testtools>=0.9.34
-mock
 coverage
 httmock