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* Removed the StrAndUnicode mix-in that is superseded by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

@@ -1,8 +1,10 @@
 # coding: utf-8
 from django.contrib.auth.models import User
 from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
 
 
+@python_2_unicode_compatible
 class Band(models.Model):
     name = models.CharField(max_length=100)
     bio = models.TextField()
@@ -11,7 +13,7 @@ class Band(models.Model):
     class Meta:
         ordering = ('name',)
 
-    def __unicode__(self):
+    def __str__(self):
         return self.name
 
 class Concert(models.Model):