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* Removed the StrAndUnicode mix-in that is superseded by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

@@ -11,12 +11,14 @@ from __future__ import absolute_import
 from django.db import models
 
 from ..model_inheritance.models import NamedURL
+from django.utils.encoding import python_2_unicode_compatible
 
 #
 # Abstract base classes with related models
 #
+@python_2_unicode_compatible
 class Copy(NamedURL):
     content = models.TextField()
 
-    def __unicode__(self):
+    def __str__(self):
         return self.content