[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* Removed the StrAndUnicode mix-in that is superseded by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/m2m_recursive/models.py

@@ -17,12 +17,14 @@ appropriate.
 """
 
 from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
 
 
+@python_2_unicode_compatible
 class Person(models.Model):
     name = models.CharField(max_length=20)
     friends = models.ManyToManyField('self')
     idols = models.ManyToManyField('self', symmetrical=False, related_name='stalkers')
 
-    def __unicode__(self):
+    def __str__(self):
         return self.name