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

openedx/core/djangoapps/content/course_overviews/tests/test_api.py

@@ -1,7 +1,7 @@
 """
 course_overview api tests
 """
-from mock import patch
+from unittest.mock import patch
 
 from django.http.response import Http404
 from opaque_keys.edx.keys import CourseKey