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: I8757d430ab233f4a21d1d599f1a7a9a0dc68d7ef

.zuul.yaml

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

swift_manager/test_manager.py

@@ -1,4 +1,4 @@
-import mock
+from unittest import mock
 import unittest
 
 import manager

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_actions.py

@@ -17,11 +17,11 @@ import sys
 import tempfile
 import subprocess
 
-import mock
+from unittest import mock
 import yaml
 import unittest
 
-from mock import patch, MagicMock, call
+from unittest.mock import patch, MagicMock, call
 
 # python-apt is not installed as part of test-requirements but is imported by
 # some charmhelpers modules so create a fake import.

unit_tests/test_actions_openstack_upgrade.py

@@ -17,7 +17,7 @@ import sys
 
 import unittest
 
-from mock import patch, MagicMock
+from unittest.mock import patch, MagicMock
 
 os.environ['JUJU_UNIT_NAME'] = 'swift-proxy'
 

unit_tests/test_swift_context.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import mock
+from unittest import mock
 import os
 import tempfile
 import unittest

unit_tests/test_swift_hooks.py

@@ -19,7 +19,7 @@ import uuid
 
 import unittest
 
-from mock import (
+from unittest.mock import (
     call,
     patch,
     MagicMock,

unit_tests/test_swift_utils.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 import copy
-import mock
+from unittest import mock
 import os
 import shutil
 import tempfile

unit_tests/test_templates.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import mock
+from unittest import mock
 import unittest
 
 from jinja2 import Environment