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

scripts/user_retirement/tests/utils/test_edx_api.py

@@ -2,12 +2,12 @@
 Tests for edX API calls.
 """
 import unittest
+from unittest.mock import DEFAULT, patch
 from urllib.parse import urljoin
 
 import requests
 import responses
 from ddt import data, ddt, unpack
-from mock import DEFAULT, patch
 from requests.exceptions import ConnectionError, HTTPError
 from responses import GET, PATCH, POST, matchers
 from responses.registries import OrderedRegistry