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* Removed the StrAndUnicode mix-in that is superseded by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

@@ -8,17 +8,19 @@ xpected results
 from __future__ import unicode_literals
 
 from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
 
 
 # The first two models represent a very simple null FK ordering case.
 class Author(models.Model):
     name = models.CharField(max_length=150)
 
+@python_2_unicode_compatible
 class Article(models.Model):
     title = models.CharField(max_length=150)
     author = models.ForeignKey(Author, null=True)
 
-    def __unicode__(self):
+    def __str__(self):
         return 'Article titled: %s' % (self.title, )
 
     class Meta:
@@ -33,13 +35,15 @@ class Forum(models.Model):
     system_info = models.ForeignKey(SystemInfo)
     forum_name = models.CharField(max_length=32)
 
+@python_2_unicode_compatible
 class Post(models.Model):
     forum = models.ForeignKey(Forum, null=True)
     title = models.CharField(max_length=32)
 
-    def __unicode__(self):
+    def __str__(self):
         return self.title
 
+@python_2_unicode_compatible
 class Comment(models.Model):
     post = models.ForeignKey(Post, null=True)
     comment_text = models.CharField(max_length=250)
@@ -47,5 +51,5 @@ class Comment(models.Model):
     class Meta:
         ordering = ['post__forum__system_info__system_name', 'comment_text']
 
-    def __unicode__(self):
+    def __str__(self):
         return self.comment_text