Use unittest.mock instead of PyPI mock

Now that py2 is gone, oslotest dropped dependency on mock and will
soon affect Ussuri CI [1], let's use unittest.mock built in py3.

This also fixes py38 jobs and proactively prevents py36 and py37
failing due to [1]. This is because we never included mock in
test-requirements and instead relied on oslotest to bring it in.

[1] https://review.opendev.org/716322

Change-Id: I0c18b13c4e1fbaa9db41da4e2039ad908c28caa6

kayobe/tests/unit/cli/test_commands.py

@@ -13,10 +13,10 @@
 # under the License.
 
 import unittest
+from unittest import mock
 
 import cliff.app
 import cliff.commandmanager
-import mock
 
 from kayobe import ansible
 from kayobe.cli import commands

kayobe/tests/unit/test_ansible.py

@@ -19,8 +19,7 @@ import shutil
 import subprocess
 import tempfile
 import unittest
-
-import mock
+from unittest import mock
 
 from kayobe import ansible
 from kayobe import exception

kayobe/tests/unit/test_kolla_ansible.py

@@ -16,8 +16,7 @@ import argparse
 import os
 import subprocess
 import unittest
-
-import mock
+from unittest import mock
 
 from kayobe import kolla_ansible
 from kayobe import utils

kayobe/tests/unit/test_utils.py

@@ -15,8 +15,8 @@
 import os
 import subprocess
 import unittest
+from unittest import mock
 
-import mock
 import yaml
 
 from kayobe import utils

kayobe/tests/unit/test_vault.py

@@ -15,8 +15,7 @@
 import argparse
 import os
 import unittest
-
-import mock
+from unittest import mock
 
 from kayobe import utils
 from kayobe import vault

tools/sphinx8

@@ -11,9 +11,9 @@ directives when any of the directives modules are imported.
 """
 
 import sys
+from unittest import mock
 
 import doc8.main
-import mock
 import sphinx.directives
 import sphinx.directives.code
 import sphinx.directives.patches