Fixed #19414 -- Added admin registration decorator

Thanks stavros for the suggestion.

tests/admin_registration/models.py

@@ -8,9 +8,15 @@ from django.db import models
 class Person(models.Model):
     name = models.CharField(max_length=200)
 
+
+class Traveler(Person):
+    pass
+
+
 class Location(models.Model):
     class Meta:
         abstract = True
 
+
 class Place(Location):
     name = models.CharField(max_length=200)