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: Ie3c9acbcfd21a7ef02c48dace486e80c502f0833
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

doc/requirements.txt

@@ -12,4 +12,3 @@ sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2;python_version>='3.4' # BSD
 sphinxcontrib-apidoc>=0.2.0  # BSD
 
 # NOTE: The following are required as horizon.test.settings loads it.
-mock>=2.0.0 # BSD

magnum_ui/test/api_tests/test_rest_api.py

@@ -12,7 +12,7 @@
 # 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
 

magnum_ui/test/helpers.py

@@ -12,7 +12,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import mock
+from unittest import mock
 
 from magnum_ui import api
 from magnum_ui.test import test_data

test-requirements.txt

@@ -9,7 +9,6 @@
 # Hacking should appear first in case something else depends on pep8
 hacking>=1.1.0,<1.2.0 # Apache-2.0
 coverage!=4.4,>=4.0 # Apache-2.0
-mock>=2.0.0 # BSD
 nodeenv>=0.9.4 # BSD
 pytest>=5.3.5 # MIT
 selenium>=2.50.1 # Apache-2.0