unittest: use unittest.mock instead of mock

because the absence of unittest.mock on python2.7, we still fallback to
mock

tests/test_cli/test_import_comps.py

@@ -7,7 +7,10 @@ import json
 import pytest
 import koji
 
-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 import six
 
 try: