[stable-2.13] Use unittest.mock instead of mock. (#77883)

This can only be done for controller tests.
(cherry picked from commit 3a9a23fb1a)

Co-authored-by: Matt Clay <matt@mystile.com>

test/units/plugins/callback/test_callback.py

@@ -25,7 +25,7 @@ import textwrap
 import types
 
 from units.compat import unittest
-from mock import MagicMock
+from unittest.mock import MagicMock
 
 from ansible.executor.task_result import TaskResult
 from ansible.inventory.host import Host