Replace Manager with BaseManager

[intgr]

@flaeppe This is some example fallout from typeddjango/django-stubs#1814 since RelatedManager base class was changed from Manager to BaseManager.

Without this change, I can no longer pass RelatedManagers to APIs that accept them.

I'm not entirely happy or sure about this. I think in pratice most managers, including RelatedManagers, do inherit from Manager.

BaseManager seems to be intended as an impementation detail in Django, not something users should be exposed to. But this will force users to replace Manager type hints with BaseManager, including my personal projects.

Related issues

• Move RelatedManager to django.db.models.fields.related_descriptors django-stubs#1814

[intgr]

Closing this, looks like we'll be reversing the change in django-stubs instead:

• Restore RelatedManager, ManyRelatedManager to inherit from Manager not BaseManager django-stubs#1843