[1.7.x] Further consolidated the model_inheritance tests.

Backport of 3f01e82 from master

tests/model_inheritance_regress/models.py

@@ -73,9 +73,14 @@ class ParkingLot4B(Place, ParkingLot4):
     pass
 
 
+@python_2_unicode_compatible
 class Supplier(models.Model):
+    name = models.CharField(max_length=50)
     restaurant = models.ForeignKey(Restaurant)
 
+    def __str__(self):
+        return self.name
+
 
 class Wholesaler(Supplier):
     retailer = models.ForeignKey(Supplier, related_name='wholesale_supplier')