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* Removed the StrAndUnicode mix-in that is superseded by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

@@ -8,26 +8,29 @@ table creation or queries.
 from __future__ import unicode_literals
 
 from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
 
 
+@python_2_unicode_compatible
 class Article(models.Model):
     Article_ID = models.AutoField(primary_key=True, db_column='Article ID')
     headline = models.CharField(max_length=100)
     authors = models.ManyToManyField('Author', db_table='my m2m table')
     primary_author = models.ForeignKey('Author', db_column='Author ID', related_name='primary_set')
 
-    def __unicode__(self):
+    def __str__(self):
         return self.headline
 
     class Meta:
         ordering = ('headline',)
 
+@python_2_unicode_compatible
 class Author(models.Model):
     Author_ID = models.AutoField(primary_key=True, db_column='Author ID')
     first_name = models.CharField(max_length=30, db_column='first name')
     last_name = models.CharField(max_length=30, db_column='last name')
 
-    def __unicode__(self):
+    def __str__(self):
         return '%s %s' % (self.first_name, self.last_name)
 
     class Meta: