[FR] Make django.contrib.auth.base_user._UserModel part of a public API

[sterliakov]

Introduced in #2335, _UserModel type alias is resolved by plugin to point to current AUTH_USER_MODEL.

This functionality is extremely helpful in certain circumstances, esp. when typing django-related packages. I was trying to improve type hints of djangorestframework-simplejwt and noticed that it'd be very convenient to use this _UserModel alias to spell "type of current user model".

For example, this will help me annotate the authentication class correctly: it calls get_user_model internally, assigning returned class to an instance variable. Annotating with _UserModel would greatly improve usability compared to AbstractBaseUser.

I propose to add an alias to django_stubs_ext resolved in the same way as _UserModel.

I'm willing to open a PR with this change if that sounds reasonable.

[flaeppe]

Yes I agree, we should make it public. Having _UserModel public should be very helpful for 3rd party code.

We've previously mentioned this in: typeddjango/djangorestframework-stubs#650 (comment) and typeddjango/djangorestframework-stubs#651 (review)