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.

Note that https://github.com/openstack/charms.openstack is used during tests
and he need `mock`, unfortunatelly it doesn't declare `mock` in its
requirements so it retrieve mock from other charm project (cross dependency).
So we depend on charms.openstack first and when
Ib1ed5b598a52375e29e247db9ab4786df5b6d142 will be merged then CI
will pass without errors.

Depends-On: Ib1ed5b598a52375e29e247db9ab4786df5b6d142
Change-Id: I57ef29855eefd0f8dc40cc427eb9bfc8796032e3

.zuul.yaml

@@ -1,5 +1,4 @@
 - project:
     templates:
-      - python35-charm-jobs
       - openstack-python3-ussuri-jobs
       - openstack-cover-jobs

test-requirements.txt

@@ -13,11 +13,6 @@ setuptools<50.0.0  # https://github.com/pypa/setuptools/commit/04e3df22df840c6bb
 
 requests>=2.18.4
 
-# Newer mock seems to have some syntax which is newer than python3.5 (e.g.
-# f'{something}'
-mock>=1.2,<4.0.0; python_version < '3.6'
-mock>=1.2; python_version >= '3.6'
-
 stestr>=2.2.0
 
 # Dependency of stestr. Workaround for

unit_tests/test_glance_simplestreams_sync.py

@@ -17,7 +17,7 @@ limitations under the License.
 '''
 
 import files.glance_simplestreams_sync as gss
-import mock
+import unittest.mock as mock
 import unittest
 
 from keystoneclient import exceptions as keystone_exceptions

unit_tests/test_hooks.py

@@ -1,5 +1,5 @@
 import base64
-import mock
+from unittest import mock
 import os
 import shutil
 import tempfile

unit_tests/test_utils.py

@@ -4,7 +4,7 @@ import os
 import yaml
 
 from contextlib import contextmanager
-from mock import patch, MagicMock
+from unittest.mock import patch, MagicMock
 
 
 def load_config():