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* Removed the StrAndUnicode mix-in that is superseded by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

@@ -1,6 +1,7 @@
 from django.contrib.contenttypes import generic
 from django.contrib.contenttypes.models import ContentType
 from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
 
 
 class Episode(models.Model):
@@ -9,6 +10,7 @@ class Episode(models.Model):
     author = models.CharField(max_length=100, blank=True)
 
 
+@python_2_unicode_compatible
 class Media(models.Model):
     """
     Media that can associated to any object.
@@ -20,7 +22,7 @@ class Media(models.Model):
     description = models.CharField(max_length=100, blank=True)
     keywords = models.CharField(max_length=100, blank=True)
 
-    def __unicode__(self):
+    def __str__(self):
         return self.url
 
 #