BaseBackend.get_user: Relax user_id param type (#2121)

In theory, this pk could be anything, given that `AUTH_USER_MODEL` can resolved
to any custom model (often a `AbstractBaseUser` subclass.)

See also:
- https://docs.djangoproject.com/en/5.0/topics/auth/customizing/#substituting-a-custom-user-model
- https://github.com/django/django/blob/5.0.4/django/contrib/auth/backends.py

django-stubs/contrib/auth/backends.pyi

@@ -13,7 +13,7 @@ UserModel: Any
 
 class BaseBackend:
     def authenticate(self, request: HttpRequest | None, **kwargs: Any) -> AbstractBaseUser | None: ...
-    def get_user(self, user_id: int) -> AbstractBaseUser | None: ...
+    def get_user(self, user_id: Any) -> AbstractBaseUser | None: ...
     def get_user_permissions(self, user_obj: _AnyUser, obj: Model | None = ...) -> set[str]: ...
     def get_group_permissions(self, user_obj: _AnyUser, obj: Model | None = ...) -> set[str]: ...
     def get_all_permissions(self, user_obj: _AnyUser, obj: Model | None = ...) -> set[str]: ...