fix: UserAdmin accepts AbstractUser (#2139)

Co-authored-by: Nikita Sobolev <[email protected]>

django-stubs/contrib/auth/admin.pyi

@@ -1,16 +1,18 @@
-from typing import Any
+from typing import Any, TypeVar
 
 from django.contrib import admin
-from django.contrib.auth.models import Group, User
+from django.contrib.auth.models import AbstractUser, Group
 from django.http.request import HttpRequest
 from django.http.response import HttpResponse
 
+_AbstractUserT = TypeVar("_AbstractUserT", bound=AbstractUser)
+
 csrf_protect_m: Any
 sensitive_post_parameters_m: Any
 
 class GroupAdmin(admin.ModelAdmin[Group]): ...
 
-class UserAdmin(admin.ModelAdmin[User]):
+class UserAdmin(admin.ModelAdmin[_AbstractUserT]):
     change_user_password_template: Any
     add_fieldsets: Any
     add_form: Any