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: I3b728aee2f07ad547ee75cabf78659cef1233bc5
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

barbicanclient/tests/test_client.py

@@ -13,8 +13,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from unittest import mock
+
 from keystoneauth1 import session
-import mock
 from requests_mock.contrib import fixture
 import testtools
 

barbicanclient/tests/v1/test_containers.py

@@ -13,7 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import mock
+from unittest import mock
+
 from oslo_serialization import jsonutils
 from oslo_utils import timeutils
 import six

test-requirements.txt

@@ -5,7 +5,6 @@ coverage!=4.4,>=4.1 # Apache-2.0
 hacking>=1.1.0,<1.2.0 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD
 requests-mock>=1.2.0 # Apache-2.0
-mock>=2.0.0 # BSD
 stestr>=2.0.0 # Apache-2.0
 testtools>=2.2.0 # MIT
 oslotest>=3.2.0 # Apache-2.0