  
      def test_overriding_prefetch(self):
          with self.assertNumQueries(3):
              qs = Author.objects.prefetch_related('books', 'books__read_by')
              lists = [[[six.text_type(r) for r in b.read_by.all()]
                        for b in a.books.all()]
                       for a in qs]
              self.assertEqual(lists, [
                  [["Amy"], ["Belinda"]],  # Charlotte - Poems, Jane Eyre
                  [["Amy"]],                # Anne - Poems
 
</s> remove             lists = [[[six.text_type(r) for r in b.read_by.all()]
 </s> add             lists = [[[str(r) for r in b.read_by.all()] </s> remove             lists = [[[six.text_type(r) for r in b.read_by.all()]
 </s> add             lists = [[[str(r) for r in b.read_by.all()] </s> remove             lists = [[six.text_type(r) for r in b.read_by.all()] for b in author.books.all()]
 </s> add             lists = [[str(r) for r in b.read_by.all()] for b in author.books.all()] </s> remove             lists = [[six.text_type(r) for r in a.first_book.read_by.all()]
 </s> add             lists = [[str(r) for r in a.first_book.read_by.all()] </s> remove                 [six.text_type(i_like) for i_like in author.favorite_authors.all()],
                [six.text_type(likes_me) for likes_me in author.favors_me.all()]
 </s> add                 [str(i_like) for i_like in author.favorite_authors.all()],
                [str(likes_me) for likes_me in author.favors_me.all()]
            lists = [[[str(r) for r in b.read_by.all()]
  
      def test_overriding_prefetch(self):
          with self.assertNumQueries(3):
              qs = Author.objects.prefetch_related('books', 'books__read_by')
            lists = [[[str(r) for r in b.read_by.all()]
                        for b in a.books.all()]
                       for a in qs]
              self.assertEqual(lists, [
                  [["Amy"], ["Belinda"]],  # Charlotte - Poems, Jane Eyre
                  [["Amy"]],                # Anne - Poems
keep keep keep keep replace keep keep keep keep keep
 Refs #23919 -- Removed six.<various>_types usage

Thanks Tim Graham and Simon Charette for the reviews. 
tests/prefetch_related/tests.py