Always used the library packaged mock

The library packaged mock seems to work
better across python versions, so instead of
trying to mix the system one and the library
one always prefer the library one (which should
work in all supported python versions).

Change-Id: Id6ada69df9961a6b7511316e2f75ef9ee5f0b72b

taskflow/test.py

@@ -20,23 +20,11 @@ import collections
 import logging
 
 import fixtures
+import mock
 from oslotest import base
 from oslotest import mockpatch
 import six
 
-# This is weird like this since we want to import a mock that works the best
-# and we need to try this import order, since oslotest registers a six.moves
-# module (but depending on the import order of importing oslotest we may or
-# may not see that change when trying to use it from six).
-try:
-    from six.moves import mock
-except ImportError:
-    try:
-        # In python 3.3+ mock got included in the standard library...
-        from unittest import mock
-    except ImportError:
-        import mock
-
 from testtools import compat
 from testtools import matchers
 from testtools import testcase