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* Removed the StrAndUnicode mix-in that is superseded by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

@@ -2,47 +2,54 @@
 from __future__ import unicode_literals
 
 from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
 
 
+@python_2_unicode_compatible
 class Foo(models.Model):
     name = models.CharField(max_length=50)
     friend = models.CharField(max_length=50, blank=True)
 
-    def __unicode__(self):
+    def __str__(self):
         return "Foo %s" % self.name
 
+@python_2_unicode_compatible
 class Bar(models.Model):
     name = models.CharField(max_length=50)
     normal = models.ForeignKey(Foo, related_name='normal_foo')
     fwd = models.ForeignKey("Whiz")
     back = models.ForeignKey("Foo")
 
-    def __unicode__(self):
+    def __str__(self):
         return "Bar %s" % self.place.name
 
+@python_2_unicode_compatible
 class Whiz(models.Model):
     name = models.CharField(max_length=50)
 
-    def __unicode__(self):
+    def __str__(self):
         return "Whiz %s" % self.name
 
+@python_2_unicode_compatible
 class Child(models.Model):
     parent = models.OneToOneField('Base')
     name = models.CharField(max_length=50)
 
-    def __unicode__(self):
+    def __str__(self):
         return "Child %s" % self.name
 
+@python_2_unicode_compatible
 class Base(models.Model):
     name = models.CharField(max_length=50)
 
-    def __unicode__(self):
+    def __str__(self):
         return "Base %s" % self.name
 
+@python_2_unicode_compatible
 class Article(models.Model):
     name = models.CharField(max_length=50)
     text = models.TextField()
     submitted_from = models.IPAddressField(blank=True, null=True)
 
-    def __unicode__(self):
+    def __str__(self):
         return "Article %s" % self.name