convince mypy that user.is_staff (and friends) are booleans (#542)

closes #512

Co-authored-by: proxi <51172302+3n-k1@users.noreply.github.com>

tests/typecheck/contrib/auth/test_misc.yml

@@ -13,3 +13,25 @@
         from django.test import Client
 
         get_user(Client())
+-   case: test_user_fields
+    main: |
+        from typing import Union
+        from django.contrib.auth.models import AnonymousUser, User
+
+        anonymous: AnonymousUser
+        anonymous_is_staff: bool = anonymous.is_staff
+        anonymous_is_active: bool = anonymous.is_active
+        anonymous_is_superuser: bool = anonymous.is_superuser
+        anonymous_is_authenticated: bool = anonymous.is_authenticated
+
+        user: User
+        user_is_staff: bool = user.is_staff
+        user_is_active: bool = user.is_active
+        user_is_superuser: bool = user.is_superuser
+        user_is_authenticated: bool = user.is_authenticated
+
+        union: Union[User, AnonymousUser]
+        union_is_staff: bool = union.is_staff
+        union_is_active: bool = union.is_active
+        union_is_superuser: bool = union.is_superuser
+        union_is_authenticated: bool = union.is_authenticated