Use unittest.mock instead of third party mock

mock has been removed from requirements files of its deps
which used to install mock for this repo gate.

Fixing gate by moving it to unittest.mock.

Change-Id: I5e0ef2a74d03333ebd9d4be1ef7fcb040e54ed30

osc_placement/tests/unit/test_http.py

@@ -10,8 +10,8 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+from unittest import mock
 
-import mock
 import six
 
 import keystoneauth1.exceptions.http as ks_exceptions

osc_placement/tests/unit/test_plugin.py

@@ -11,8 +11,8 @@
 # under the License.
 
 import argparse
+from unittest import mock
 
-import mock
 from oslotest import base
 
 import osc_placement.plugin as plugin

osc_placement/tests/unit/test_version.py

@@ -10,7 +10,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import mock
+from unittest import mock
 
 import oslotest.base as base
 import six