Use unittest.mock instead of third party mock

Now that we no longer support py27, we can use the standard library
unittest.mock module instead of the third party mock lib.

Change-Id: I564357caf5982a0db9be01c77e9e8c42bee932d6
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

test-requirements.txt

@@ -9,4 +9,3 @@ oslotest>=3.3.0 # Apache-2.0
 testrepository>=0.0.18 # Apache-2.0/BSD
 testscenarios>=0.5.0 # Apache-2.0/BSD
 testtools>=2.3.0 # MIT
-mock>=2.0.0 # BSD

vitrageclient/tests/cli/test_event_post.py

@@ -14,10 +14,10 @@
 
 from argparse import ArgumentParser
 from argparse import ArgumentTypeError
+from unittest import mock
 
 # noinspection PyPackageRequirements
 from datetime import datetime
-import mock
 from testtools import ExpectedException
 
 from vitrageclient.tests.cli.base import CliTestCase

vitrageclient/tests/cli/test_topology_show.py

@@ -13,11 +13,11 @@
 # under the License.
 from argparse import ArgumentParser
 from argparse import ArgumentTypeError
+from unittest import mock
 
 import json
 
 # noinspection PyPackageRequirements
-import mock
 # noinspection PyPackageRequirements
 import six
 from testtools import ExpectedException

vitrageclient/tests/test_template.py

@@ -12,7 +12,8 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import mock
+from unittest import mock
+
 from oslotest import base
 
 from vitrageclient import exceptions as exc