Use unittest.mock if is only aviable

from Python 3.3 is mock part of python standard library in unittest namespace

testing/example_scripts/acceptance/fixture_mock_integration.py

@@ -1,6 +1,9 @@
 """Reproduces issue #3774"""
 
+try:
     import mock
+except ImportError:
+    import unittest.mock as mock
 
 import pytest