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: I8ca08483bf40a3092275d5dd0458951a64f239a1

test-requirements.txt

@@ -2,5 +2,4 @@
 flake8
 stestr>=2.2.0
 charms.reactive
-mock>=1.2
 coverage>=3.6

unit_tests/test_requires.py

@@ -12,7 +12,7 @@
 
 
 import unittest
-import mock
+from unittest import mock
 
 import requires