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: Iaf7949e304a6e11ff0ac47875bec69c7ccb2d621

test-requirements.txt

@@ -2,5 +2,4 @@
 flake8
 os-testr>=0.4.1
 charms.reactive
-mock>=1.2
 coverage>=3.6

unit_tests/__init__.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 import sys
-import mock
+from unittest import mock
 
 # mock out some charmhelpers libraries as they have apt install side effects
 apt_pkg = mock.MagicMock()

unit_tests/test_provides.py

@@ -11,7 +11,7 @@
 # WITHOUT 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 mock
+from unittest import mock
 import yaml
 
 import unit_tests.utils as ut_utils

unit_tests/utils.py

@@ -17,7 +17,7 @@
 
 import contextlib
 import io
-import mock
+from unittest import mock
 import unittest