- Added a new class `_GenericMixin` to serve as a runtime placeholder for `Generic[ModelT]`. This change prevents `TypeError` during `__class__` assignments, which was an issue when mixins inherited from `Generic[T]` at runtime.
- All manager mixins have been updated to inherit from `_GenericMixin` instead of `Generic[ModelT]`. This ensures compatibility with `django-modeltranslation`.
- Introduced regressions tests to confirm that the manager instances support `__class__` reassignment without issues. Tests were added for `SoftDeletableManager`, `InheritanceManager`, `QueryManager`, and `JoinManager`.
Closes GH-#636.
