[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* Removed the StrAndUnicode mix-in that is superseded by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/modeltests/get_object_or_404/models.py

@@ -11,23 +11,26 @@ performing a ``filter()`` lookup and raising a ``Http404`` exception if a
 """
 
 from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
 
 
+@python_2_unicode_compatible
 class Author(models.Model):
     name = models.CharField(max_length=50)
 
-    def __unicode__(self):
+    def __str__(self):
         return self.name
 
 class ArticleManager(models.Manager):
     def get_query_set(self):
         return super(ArticleManager, self).get_query_set().filter(authors__name__icontains='sir')
 
+@python_2_unicode_compatible
 class Article(models.Model):
     authors = models.ManyToManyField(Author)
     title = models.CharField(max_length=50)
     objects = models.Manager()
     by_a_sir = ArticleManager()
 
-    def __unicode__(self):
+    def __str__(self):
         return self.title