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: I64f6f5df4d82faef205e5019be5a41c1b1f1f1fa
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

ceilometermiddleware/tests/test_swift.py

@@ -13,8 +13,8 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 import threading
+from unittest import mock
 
-import mock
 from oslo_config import cfg
 import six
 

test-requirements.txt

@@ -8,6 +8,5 @@ sphinx!=1.6.6,!=1.6.7,!=2.1.0 # BSD
 openstackdocstheme>=1.17.0  # Apache-2.0
 oslotest>=1.10.0 # Apache-2.0
 stestr>=2.0.0 # Apache-2.0
-mock>=1.2 # BSD
 reno>=0.1.1 # Apache-2.0
 betamax>=0.7.0 # Apache-2.0