Cleanup use of mock package in for python2

Mock is part of unittest in python3 and now that zuul is a python3 only
application we can cleanup the package dependency on mock and the
fallback imports that were done to support python2.

Change-Id: I55a9831b96bdc384ff04723b16c8af42084af03c

test-requirements.txt

@@ -10,7 +10,6 @@ testtools>=0.9.32
 sphinxcontrib-programoutput
 sphinx-autodoc-typehints
 reno>=2.5.0 # Apache-2.0
-mock
 PyMySQL
 mypy
 zuul-sphinx

tests/unit/test_executor.py

@@ -15,13 +15,9 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-try:
-    from unittest import mock
-except ImportError:
-    import mock
-
 import logging
 import time
+from unittest import mock
 
 import zuul.executor.server
 import zuul.model

tests/unit/test_gerrit.py

@@ -13,12 +13,9 @@
 # WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 # License for the specific language governing permissions and limitations
 # under the License.
-import os
 
-try:
-    from unittest import mock
-except ImportError:
-    import mock
+import os
+from unittest import mock
 
 import tests.base
 from tests.base import BaseTestCase