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* Removed the StrAndUnicode mix-in that is superseded by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

@@ -12,6 +12,7 @@ returns.
 from __future__ import unicode_literals
 
 from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
 
 # An example of a custom manager called "objects".
 
@@ -19,13 +20,14 @@ class PersonManager(models.Manager):
     def get_fun_people(self):
         return self.filter(fun=True)
 
+@python_2_unicode_compatible
 class Person(models.Model):
     first_name = models.CharField(max_length=30)
     last_name = models.CharField(max_length=30)
     fun = models.BooleanField()
     objects = PersonManager()
 
-    def __unicode__(self):
+    def __str__(self):
         return "%s %s" % (self.first_name, self.last_name)
 
 # An example of a custom manager that sets get_query_set().
@@ -34,6 +36,7 @@ class PublishedBookManager(models.Manager):
     def get_query_set(self):
         return super(PublishedBookManager, self).get_query_set().filter(is_published=True)
 
+@python_2_unicode_compatible
 class Book(models.Model):
     title = models.CharField(max_length=50)
     author = models.CharField(max_length=30)
@@ -41,7 +44,7 @@ class Book(models.Model):
     published_objects = PublishedBookManager()
     authors = models.ManyToManyField(Person, related_name='books')
 
-    def __unicode__(self):
+    def __str__(self):
         return self.title
 
 # An example of providing multiple custom managers.
@@ -50,6 +53,7 @@ class FastCarManager(models.Manager):
     def get_query_set(self):
         return super(FastCarManager, self).get_query_set().filter(top_speed__gt=150)
 
+@python_2_unicode_compatible
 class Car(models.Model):
     name = models.CharField(max_length=10)
     mileage = models.IntegerField()
@@ -57,5 +61,5 @@ class Car(models.Model):
     cars = models.Manager()
     fast_cars = FastCarManager()
 
-    def __unicode__(self):
+    def __str__(self):
         return self.name