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: I4c7cb63e0a816b361c2544b1be34d8a6dadeb5c0

lower-constraints.txt

@@ -43,7 +43,6 @@ linecache2==1.0.0
 Mako==0.4.0
 MarkupSafe==1.0
 mccabe==0.2.1
-mock==2.0.0
 monasca-common==2.16.0
 monotonic==0.6
 mox3==0.20.0

monasca_api/tests/test_alarms.py

@@ -24,7 +24,7 @@ import falcon.testing
 import fixtures
 import testtools.matchers as matchers
 
-from mock import Mock
+from unittest.mock import Mock
 
 import oslo_config.fixture
 import six

monasca_api/tests/test_query_helpers.py

@@ -14,7 +14,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-from mock import Mock
+from unittest.mock import Mock
 
 from monasca_api.tests import base
 from monasca_api.v2.reference import helpers

monasca_api/tests/test_repositories.py

@@ -21,7 +21,7 @@ from datetime import datetime
 
 import cassandra
 from cassandra.auth import PlainTextAuthProvider
-from mock import patch
+from unittest.mock import patch
 
 from oslo_config import cfg
 from oslo_utils import timeutils