Django 5.2 made an un-announced change that moved fixture loading to

happen at class init time rather than per-test-method. As part of this,

the _load_fixtures method has been made a @classmethod. This is a

fundamentally better approach, but causes numerous problems with our own

fixture code.
This change updates our FixturesCompilerMixin to operate at class-time

rather than at test function time. This requires changing the way we

hide the fixtures classvar from the parent class, since

__getattribute__ can't work.
The @add_fixtures decorator is now a performance hit rather than a

performance benefit. In addition, the replace argument is no longer

possible (but luckily, nothing actually used that). I've added a warning

about the performance implications, but for now it's hidden behind an

environment variable because we use this decorator a lot.

Ran unit tests and verified that fixtures worked appropriately.