add proper stubs for django.contrib.auth.models model fields (#170)

test-data/typecheck/models/test_contrib_models.yml

@@ -0,0 +1,22 @@
+-   case: contrib_auth_model_fields
+    main: |
+        from django.contrib.auth.models import User
+        reveal_type(User().username)  # N: Revealed type is 'builtins.str*'
+        reveal_type(User().password)  # N: Revealed type is 'builtins.str*'
+        reveal_type(User().first_name)  # N: Revealed type is 'builtins.str*'
+        reveal_type(User().last_name)  # N: Revealed type is 'builtins.str*'
+        reveal_type(User().email)  # N: Revealed type is 'builtins.str*'
+        reveal_type(User().is_staff)  # N: Revealed type is 'builtins.bool*'
+        reveal_type(User().is_active)  # N: Revealed type is 'builtins.bool*'
+        reveal_type(User().date_joined)  # N: Revealed type is 'datetime.datetime*'
+        reveal_type(User().last_login)  # N: Revealed type is 'Union[datetime.datetime, None]'
+
+        from django.contrib.auth.models import Permission
+        reveal_type(Permission().name)  # N: Revealed type is 'builtins.str*'
+        reveal_type(Permission().codename)  # N: Revealed type is 'builtins.str*'
+
+        from django.contrib.auth.models import PermissionsMixin
+        reveal_type(PermissionsMixin().is_superuser)  # N: Revealed type is 'builtins.bool*'
+
+        from django.contrib.auth.models import Group
+        reveal_type(Group().name)  # N: Revealed type is 'builtins.str*'