[py3] Refactored __unicode__ to __str__.

* Renamed the __unicode__ methods
* Applied the python_2_unicode_compatible decorator
* Removed the StrAndUnicode mix-in that is superseded by
  python_2_unicode_compatible
* Kept the __unicode__ methods in classes that specifically
  test it under Python 2

tests/regressiontests/model_inheritance_select_related/models.py

@@ -5,27 +5,31 @@ select_related().
 from __future__ import unicode_literals
 
 from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
 
 
+@python_2_unicode_compatible
 class Place(models.Model):
     name = models.CharField(max_length=50)
 
     class Meta:
         ordering = ('name',)
 
-    def __unicode__(self):
+    def __str__(self):
         return "%s the place" % self.name
 
+@python_2_unicode_compatible
 class Restaurant(Place):
     serves_sushi = models.BooleanField()
     serves_steak = models.BooleanField()
 
-    def __unicode__(self):
+    def __str__(self):
         return "%s the restaurant" % self.name
 
+@python_2_unicode_compatible
 class Person(models.Model):
     name = models.CharField(max_length=50)
     favorite_restaurant = models.ForeignKey(Restaurant)
 
-    def __unicode__(self):
+    def __str__(self):
         return self.name