Fix compatibility with django-polymorphic 4.6 (complete)

[djmaze]

As the author of #279 is not responsive, here is a new PR which should fix the remaining problems.

[nattyg93]

@fdintino Any chance we can get this merged and released? With Django 4.2 reaching EOL, this is a blocker for upgrading to Django 5.2 when using django-polymorphic and django-nested-admin.

[fdintino]

The reason I haven't merged this yet is that the logic here is very different than what was in _get_inheritance_relation_fields_and_models.

https://github.com/jazzband/django-polymorphic/blob/d2f9ca27c0bd0ca522e80266f5205a760d7f74d2/src/polymorphic/models.py#L342-L390

I would be more inclined to merge this if the logic was functionally identical. At the very least it needs to be using __subclasses__ since this variable is meant to include child models only. The model parents are already accounted for in a different variable. I think this might be functionally equivalent:

model_cls = self.formset.model.__class__
sub_models = [
    sub_cls
    for sub_cls in model_cls.__subclasses__()
    if model_cls in sub_cls._meta.parents
]

[djmaze]

The reason I haven't merged this yet is that the logic here is very different than what was in _get_inheritance_relation_fields_and_models.

@fdintino Okay, sorry, I was not knowledgeable in this codebase. I just tried to implement your suggested approach, maybe it is more correct now?