          form_data = {
              'headline': 'New headline',
              'slug': 'new-headline',
              'pub_date': '1988-01-04',
              'writer': six.text_type(self.w_royko.pk),
              'article': 'Hello.',
              'categories': [six.text_type(self.c1.id), six.text_type(self.c2.id)]
          }
          # Create a new article, with categories, via the form.
          f = ArticleForm(form_data)
 
</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_royko.pk),
 </s> add             'writer': str(self.w_royko.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             'writer': six.text_type(self.w_woodward.pk),
 </s> add             'writer': str(self.w_woodward.pk), </s> remove             'host_set-0-id': six.text_type(host1.id),
 </s> add             'host_set-0-id': str(host1.id),
            'writer': str(self.w_royko.pk),
          form_data = {
              'headline': 'New headline',
              'slug': 'new-headline',
              'pub_date': '1988-01-04',
            'writer': str(self.w_royko.pk),
              'article': 'Hello.',
              'categories': [six.text_type(self.c1.id), six.text_type(self.c2.id)]
          }
          # Create a new article, with categories, via the form.
          f = ArticleForm(form_data)
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