django.contrib.auth: Relax ModelBackend -> BaseBackend (#2141)

`BaseBackend` is the base interface of django backends, `ModelBackend`
has more specific signatures, such as `.authenticate()` including
`username` and `password`, not all authentication backends use those.

For instance, since the `authenticate()` method in BaseBackend already includes
request (`HttpRequest`), it can read a `Bearer` token or cookie, and already accepts
`**kwargs` for additional parameters.

See also:
- BaseBackend: https://github.com/django/django/blob/5.0.6/django/contrib/auth/backends.py#L8-L28
  - ModelBackend: https://github.com/django/django/blob/5.0.6/django/contrib/auth/backends.py#L31-L160
- django.contrib.auth.authenticate(): https://github.com/django/django/blob/5.0.6/django/contrib/auth/__init__.py#L65-L93
- Django Docs for "Writing an authentication backend": https://docs.djangoproject.com/en/5.0/topics/auth/customizing/#writing-an-authentication-backend

django-stubs/contrib/auth/__init__.pyi

@@ -1,6 +1,6 @@
 from typing import Any
 
-from django.contrib.auth.backends import ModelBackend
+from django.contrib.auth.backends import BaseBackend
 from django.contrib.auth.base_user import AbstractBaseUser
 from django.contrib.auth.models import AnonymousUser
 from django.db.models.options import Options
@@ -16,15 +16,15 @@ BACKEND_SESSION_KEY: str
 HASH_SESSION_KEY: str
 REDIRECT_FIELD_NAME: str
 
-def load_backend(path: str) -> ModelBackend: ...
-def get_backends() -> list[ModelBackend]: ...
+def load_backend(path: str) -> BaseBackend: ...
+def get_backends() -> list[BaseBackend]: ...
 def authenticate(request: HttpRequest | None = ..., **credentials: Any) -> AbstractBaseUser | None: ...
 async def aauthenticate(request: HttpRequest | None = ..., **credentials: Any) -> AbstractBaseUser | None: ...
 def login(
-    request: HttpRequest, user: AbstractBaseUser | None, backend: type[ModelBackend] | str | None = ...
+    request: HttpRequest, user: AbstractBaseUser | None, backend: type[BaseBackend] | str | None = ...
 ) -> None: ...
 async def alogin(
-    request: HttpRequest, user: AbstractBaseUser | None, backend: type[ModelBackend] | str | None = ...
+    request: HttpRequest, user: AbstractBaseUser | None, backend: type[BaseBackend] | str | None = ...
 ) -> None: ...
 def logout(request: HttpRequest) -> None: ...
 async def alogout(request: HttpRequest) -> None: ...