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/thirdparty_apis/test_salesforce.py

@@ -3,8 +3,8 @@ Tests for the Salesforce API functionality
 """
 import logging
 from contextlib import contextmanager
+from unittest import mock
 
-import mock
 import pytest
 from simple_salesforce import SalesforceError
 

scripts/user_retirement/tests/utils/thirdparty_apis/test_segment_api.py

@@ -2,8 +2,8 @@
 Tests for the Segment API functionality
 """
 import json
+from unittest import mock
 
-import mock
 import pytest
 import requests
 from six import text_type