PR#4328: use unittest.mock instead of mock

Merges #4328
https://pagure.io/koji/pull-request/4328

Fixes: #4327
https://pagure.io/koji/issue/4327
Import mock from unittest

tests/test_builder/test_newRepo.py

@@ -1,4 +1,7 @@
 from __future__ import absolute_import
+try:
+    from unittest import mock
+except ImportError:
     import mock
 import os
 import rpm

tests/test_kojira/test_main.py

@@ -7,6 +7,9 @@ import tempfile
 import time
 import unittest
 
+try:
+    from unittest import mock
+except ImportError:
     import mock
 import pytest
 

tests/test_plugins/test_builder_scmpolicy.py

@@ -1,4 +1,7 @@
 import unittest
+try:
+    from unittest import mock
+except ImportError:
     import mock
 
 import scmpolicy