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: Ib169e3deb7ddb2bc93a206ebec4043552281aa7f

taskflow/test.py

@@ -16,9 +16,9 @@
 
 import collections
 import logging
+from unittest import mock
 
 import fixtures
-import mock
 from oslotest import base
 import six
 

taskflow/tests/unit/worker_based/test_proxy.py

@@ -15,10 +15,10 @@
 #    under the License.
 
 import socket
+from unittest import mock
 
 from taskflow.engines.worker_based import proxy
 from taskflow import test
-from taskflow.test import mock
 from taskflow.utils import threading_utils
 
 

test-requirements.txt

@@ -26,7 +26,6 @@ psycopg2>=2.8.0 # LGPL/ZPL
 pydotplus>=2.0.2 # MIT License
 hacking<2.1,>=2.0
 oslotest>=3.2.0 # Apache-2.0
-mock>=2.0.0 # BSD
 testtools>=2.2.0 # MIT
 testscenarios>=0.4 # Apache-2.0/BSD
 stestr>=2.0.0 # Apache-2.0

tools/state_graph.py

@@ -14,7 +14,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import mock
+from unittest import mock
 
 import optparse
 import os