[1.7.x] Fixed #22652 -- Replaced UserModel.objects with UserModel._default_manager.

Thanks alexdlaird for the report.

Backport of b68fac7e from master

django/contrib/auth/backends.py

@@ -106,14 +106,14 @@ class RemoteUserBackend(ModelBackend):
         # instead we use get_or_create when creating unknown users since it has
         # built-in safeguards for multiple threads.
         if self.create_unknown_user:
-            user, created = UserModel.objects.get_or_create(**{
+            user, created = UserModel._default_manager.get_or_create(**{
                 UserModel.USERNAME_FIELD: username
             })
             if created:
                 user = self.configure_user(user)
         else:
             try:
-                user = UserModel.objects.get_by_natural_key(username)
+                user = UserModel._default_manager.get_by_natural_key(username)
             except UserModel.DoesNotExist:
                 pass
         return user

django/contrib/auth/management/__init__.py

@@ -129,7 +129,7 @@ def create_superuser(app_config, verbosity=2, interactive=True, using=DEFAULT_DB
 
     from django.core.management import call_command
 
-    if not UserModel.objects.exists() and interactive:
+    if not UserModel._default_manager.exists() and interactive:
         msg = ("\nYou have installed Django's auth system, and "
             "don't have any superusers defined.\nWould you like to create one "
             "now? (yes/no): ")