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: Ib7c9cca06dcfe14440ab9711cc0b76a5b4fe4e1c

lower-constraints.txt

@@ -22,7 +22,6 @@ kazoo==2.2
 keystoneauth1==3.4.0
 linecache2==1.0.0
 mccabe==0.2.1
-mock==2.0.0
 monasca-common==2.16.0
 monasca-statsd==1.4.0
 monotonic==0.6

tests/test_utils.py

@@ -12,7 +12,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-from mock import patch
+from unittest.mock import patch
 
 from monasca_notification.common import utils
 from tests import base