[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* Removed the StrAndUnicode mix-in that is superseded by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/admin_custom_urls/models.py

@@ -2,13 +2,15 @@ from functools import update_wrapper
 
 from django.contrib import admin
 from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
 
 
+@python_2_unicode_compatible
 class Action(models.Model):
     name = models.CharField(max_length=50, primary_key=True)
     description = models.CharField(max_length=70)
 
-    def __unicode__(self):
+    def __str__(self):
         return self.name