Avoid importing django.test in main code

`setting_changed` is actually from `django.core.signals`, and is re-exported from `django.test.signals`. `django.test` takes 20-40ms to import, so not too much, but it's an easy win to avoid this import.

knox/settings.py

@@ -1,7 +1,7 @@
 from datetime import timedelta
 
 from django.conf import settings
-from django.test.signals import setting_changed
+from django.core.signals import setting_changed
 from rest_framework.settings import APISettings, api_settings
 
 USER_SETTINGS = getattr(settings, 'REST_KNOX', None)