fix: prefetch_related_objects requires Sequence not Iterable

[emmanuel-ferdman]

PR Summary

prefetch_related_objects and aprefetch_related_objects were typed to accept Iterable[Model], but Django's implementation uses obj_list[0] indexing internally, which requires a Sequence. Passing a set or frozenset would pass type checking but crash at runtime with TypeError: 'set' object is not subscriptable. This PR changes the parameter type to Sequence[Model] to match the actual runtime requirement.

Fixes #2948

[UnknownPlatypus]

Thanks for the pr, I've left a few comments

[UnknownPlatypus]

Thanks!