Fixed #26089 -- Removed custom user test models from public API.

Thanks to Tim Graham for the review.

tests/auth_tests/models/custom_permissions.py

@@ -4,12 +4,11 @@ Django permissions model. This allows us to check that the PermissionsMixin
 includes everything that is needed to interact with the ModelBackend.
 """
 from django.contrib.auth.models import AbstractBaseUser, PermissionsMixin
-from django.contrib.auth.tests.custom_user import (
-    CustomUserManager, RemoveGroupsAndPermissions,
-)
 from django.db import models
 from django.utils.encoding import python_2_unicode_compatible
 
+from .custom_user import CustomUserManager, RemoveGroupsAndPermissions
+
 
 class CustomPermissionsUserManager(CustomUserManager):
     def create_superuser(self, email, password, date_of_birth):