refactor: switch from mock to unittest.mock (#34844)

As of Python 3.3, the 3rd-party `mock` package has been subsumed into the
standard `unittest.mock` package. Refactoring tests to use the latter will
allow us to drop `mock` as a dependency, which is currently coming in
transitively through requirements/edx/paver.in.

We don't actually drop the `mock` dependency in this PR. That will happen
naturally in:

* https://github.com/openedx/edx-platform/pull/34830

common/test/utils.py

@@ -1,6 +1,7 @@
 """
 General testing utilities.
 """
+from unittest.mock import Mock, patch
 
 
 import functools
@@ -9,7 +10,6 @@ from contextlib import contextmanager
 
 from django.dispatch import Signal
 from markupsafe import escape
-from mock import Mock, patch
 
 
 @contextmanager