replaced unittest assertions pytest assertions (#26566)

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

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

openedx/core/djangoapps/credit/tests/test_serializers.py

openedx/core/djangoapps/credit/tests/test_services.py

openedx/core/djangoapps/credit/tests/test_signals.py

openedx/core/djangoapps/credit/tests/test_signature.py

openedx/core/djangoapps/credit/tests/test_tasks.py

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

openedx/core/djangoapps/dark_lang/tests.py

openedx/core/djangoapps/demographics/tests/test_status.py

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

openedx/core/djangoapps/django_comment_common/tests.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

...nrollments/management/tests/test_enroll_user_in_course.py

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

openedx/core/djangoapps/enrollments/tests/test_data.py