Test Django 3.2

See https://docs.djangoproject.com/en/dev/releases/3.2/

Relevant changes:

• default_app_config has been deprecated
• New setting DEFAULT_AUTO_FIELD. I set it to django.db.models.AutoField for backwards compatible but new projects will use django.db.models.BigAutoField. We could also migrate to that if we want.
• New function Queryset.alias() that can replace annotate() if you only use the annotation for filtering/sorting.
• The new database function Random() might be useful to improve the performance of randomly picking subjects from the database. Not sure though.

A nice aside: No new deprecation warnings for dependencies. However, the last round of deprecation warnings still exists for django-parler.