refactor: Ran pyupgrade on openedx/core/djangoapps/embargo (#26911)

openedx/core/djangoapps/embargo/api.py

openedx/core/djangoapps/embargo/exceptions.py

openedx/core/djangoapps/embargo/fixtures/country_codes.py

openedx/core/djangoapps/embargo/forms.py

openedx/core/djangoapps/embargo/middleware.py

openedx/core/djangoapps/embargo/migrations/0001_initial.py

...djangoapps/embargo/migrations/0002_data__add_countries.py

openedx/core/djangoapps/embargo/models.py

openedx/core/djangoapps/embargo/test_utils.py

openedx/core/djangoapps/embargo/tests/factories.py

openedx/core/djangoapps/embargo/tests/test_api.py

openedx/core/djangoapps/embargo/tests/test_forms.py

openedx/core/djangoapps/embargo/tests/test_middleware.py

openedx/core/djangoapps/embargo/tests/test_models.py

openedx/core/djangoapps/embargo/tests/test_views.py