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* Removed the StrAndUnicode mix-in that is superseded by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

@@ -6,29 +6,33 @@ more information.
 from __future__ import unicode_literals
 
 from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
 
 
+@python_2_unicode_compatible
 class Author(models.Model):
     first_name = models.CharField(max_length=30)
     last_name = models.CharField(max_length=30)
 
-    def __unicode__(self):
+    def __str__(self):
         return '%s %s' % (self.first_name, self.last_name)
 
+@python_2_unicode_compatible
 class Article(models.Model):
     author = models.ForeignKey(Author)
     headline = models.CharField(max_length=100)
 
-    def __unicode__(self):
+    def __str__(self):
         return self.headline
 
+@python_2_unicode_compatible
 class Entry(models.Model):
     title = models.CharField(max_length=250)
     body = models.TextField()
     pub_date = models.DateField()
     enable_comments = models.BooleanField()
 
-    def __unicode__(self):
+    def __str__(self):
         return self.title
 
 class Book(models.Model):