Fixed #18269 -- Applied unicode_literals for Python 3 compatibility.

Thanks Vinay Sajip for the support of his django3 branch and
Jannis Leidel for the review.

tests/regressiontests/model_inheritance_select_related/models.py

@@ -2,6 +2,7 @@
 Regression tests for the interaction between model inheritance and
 select_related().
 """
+from __future__ import unicode_literals
 
 from django.db import models
 
@@ -13,14 +14,14 @@ class Place(models.Model):
         ordering = ('name',)
 
     def __unicode__(self):
-        return u"%s the place" % self.name
+        return "%s the place" % self.name
 
 class Restaurant(Place):
     serves_sushi = models.BooleanField()
     serves_steak = models.BooleanField()
 
     def __unicode__(self):
-        return u"%s the restaurant" % self.name
+        return "%s the restaurant" % self.name
 
 class Person(models.Model):
     name = models.CharField(max_length=50)