requirements: Bump Django to 3.2.x, django-filter to 21.1

requirements: Bump Django to 3.2.x, django-filter to 21.1

It seems the ORM is now smarter and requires less JOINs that previously
in two tests. In addition, a new setting is required to ensure the type
of our primary field columns doesn't change when Django 4.0 is released.

We drop support for Django 3.1 in the process, though this doesn't have
much of a real-world impact since we still support Django 2.2, an LTS
release.

Signed-off-by: Stephen Finucane <stephen@that.guru>