Preliminary Django 6.1 support

[laymonage]

Fixes compatibility issues with current main branch of Django (6.1).

Description

• Added totally_ordered attribute to FakeQuerySet. This was introduced in django/django@08b4dfc. Wagtail's tests fail without this attribute.
• Add set_base() hook to DeferringManyRelatedManager. This was introduced in a series of commits: django/django@c3b6c29, django/django@6fc2406, django/django@12d5744, and django/django@4702b36.
  • Without this, loading fixtures will not work.
  • See https://code.djangoproject.com/ticket/27380 for more details.
  • I also added the through_defaults argument to set() to match Django. This was added in django/django@769355c, which was committed well after 2ac71a2.
  • I didn't really implement the functionality for these. Would be nice to, but for now this is just a shim to prevent modelcluster from breaking entirely.

AI usage

None