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

lms/djangoapps/instructor/tests/test_tools.py

@@ -7,6 +7,7 @@ import datetime
 import json
 import unittest
 from unittest import mock
+from unittest.mock import MagicMock, patch
 
 import pytest
 from django.contrib.auth.models import User  # lint-amnesty, pylint: disable=imported-auth-user
@@ -14,7 +15,6 @@ from django.core.exceptions import MultipleObjectsReturned
 from django.test import TestCase
 from edx_when.api import get_dates_for_course, set_dates_for_course
 from edx_when.field_data import DateLookupFieldData
-from mock.mock import MagicMock, patch
 from opaque_keys.edx.keys import CourseKey
 from pytz import UTC
 from xmodule.fields import Date