          f = ArticleForm({
              'headline': 'Test headline',
              'slug': 'test-headline',
              'pub_date': '1984-02-06',
              'writer': six.text_type(self.w_royko.pk),
              'article': 'Hello.'
          }, instance=art)
          self.assertEqual(f.errors, {})
          self.assertTrue(f.is_valid())
          test_art = f.save()
 
</s> remove             'writer': six.text_type(self.w_royko.pk),
 </s> add             'writer': str(self.w_royko.pk), </s> remove             'categories': [six.text_type(self.c1.id), six.text_type(self.c2.id)]
 </s> add             'categories': [str(self.c1.id), str(self.c2.id)] </s> remove             'writer': six.text_type(self.w_woodward.pk),
 </s> add             'writer': str(self.w_woodward.pk), </s> remove         form_data['categories'] = [six.text_type(self.c1.id), six.text_type(self.c2.id)]
 </s> add         form_data['categories'] = [str(self.c1.id), str(self.c2.id)] </s> remove         self.assertEqual(six.text_type(obj), 'foo')
 </s> add         self.assertEqual(str(obj), 'foo')
            'writer': str(self.w_royko.pk),
          f = ArticleForm({
              'headline': 'Test headline',
              'slug': 'test-headline',
              'pub_date': '1984-02-06',
            'writer': str(self.w_royko.pk),
              'article': 'Hello.'
          }, instance=art)
          self.assertEqual(f.errors, {})
          self.assertTrue(f.is_valid())
          test_art = f.save()
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/model_forms/tests.py