Upgrade DRF to latest version.

lms/djangoapps/discussion/rest_api/serializers.py

lms/djangoapps/experiments/routers.py

lms/djangoapps/experiments/tests/test_views.py

lms/djangoapps/experiments/views.py

openedx/core/djangoapps/user_api/preferences/api.py

openedx/core/lib/api/view_utils.py

requirements/edx/base.in

requirements/edx/base.txt

requirements/edx/development.txt

requirements/edx/testing.txt