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: I33cd69aca44f48af2766c92ede46ff12367160c5

.zuul.yaml

@@ -1,4 +1,3 @@
 - project:
     templates:
-      - python35-charm-jobs
       - openstack-python3-ussuri-jobs

test-requirements.txt

@@ -12,12 +12,6 @@ cffi==1.14.6; python_version < '3.6'  # cffi 1.15.0 drops support for py35.
 setuptools<50.0.0  # https://github.com/pypa/setuptools/commit/04e3df22df840c6bb244e9b27bc56750c44b7c85
 
 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_ceph.py

@@ -2,7 +2,7 @@ import collections
 import subprocess
 import unittest
 
-import mock
+from unittest import mock
 
 import ceph
 

unit_tests/test_ceph_hooks.py

@@ -1,4 +1,4 @@
-import mock
+from unittest import mock
 import sys
 
 # python-apt is not installed as part of test-requirements but is imported by

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():