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: I9d33976afc6b4d475f11f3f6b751ae7093cf2a04

lower-constraints.txt

@@ -31,7 +31,6 @@ kombu==4.1.0
 linecache2==1.0.0
 MarkupSafe==1.0
 mccabe==0.2.1
-mock==2.0.0
 monotonic==1.4
 mox3==0.25.0
 msgpack==0.5.6

muranoagent/tests/unit/executors/test_chef.py

@@ -12,12 +12,11 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from unittest import mock
-
 import fixtures
 import json
-from mock import ANY
 import os
+from unittest import mock
+from unittest.mock import ANY
 
 from muranoagent import bunch
 from muranoagent import exceptions as ex