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 (but in lower-constraints where it does not
really belong at all) and instead relied on oslotest to bring
it in.

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

Change-Id: I30e82e2d87418272a71c7ee089a8acdaf8872158

kolla_ansible/tests/unit/test_filters.py

@@ -13,9 +13,9 @@
 # under the License.
 
 import unittest
+from unittest import mock
 
 import jinja2
-import mock
 
 from kolla_ansible import exception
 from kolla_ansible import filters

lower-constraints.txt

@@ -39,7 +39,6 @@ keystoneauth1==3.4.0
 linecache2==1.0.0
 MarkupSafe==1.0
 mccabe==0.2.1
-mock==2.0.0
 monotonic==1.4
 mox3==0.25.0
 msgpack==0.5.6

tests/test_kolla_docker.py

@@ -19,11 +19,8 @@ import copy
 import imp
 import os
 import sys
-
-try:
 from unittest import mock
-except ImportError:
-    import mock
+
 from docker import errors as docker_error
 from docker.types import Ulimit
 from oslotest import base

tests/test_merge_config.py

@@ -16,9 +16,9 @@
 import imp
 import os
 import sys
+from unittest import mock
 
 from io import StringIO
-import mock
 from oslotest import base