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: Ida35e5f79ee17ef8ac6ea2186019ddd4040ec882

lower-constraints.txt

@@ -28,7 +28,6 @@ keystoneauth1==3.4.0
 linecache2==1.0.0
 MarkupSafe==1.0
 mccabe==0.2.1
-mock==2.0.0
 monotonic==0.6
 mox3==0.20.0
 msgpack-python==0.4.0

troveclient/tests/test_backups.py

@@ -13,11 +13,12 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from mock import patch
-import testtools
 from unittest import mock
+from unittest.mock import patch
 import uuid
 
+import testtools
+
 from troveclient.v1 import backups
 
 """