          # ManyToManyFields are represented by a MultipleChoiceField, ForeignKeys and any
          # fields with the 'choices' attribute are represented by a ChoiceField.
          f = ArticleForm(auto_id=False)
          self.assertHTMLEqual(
              six.text_type(f),
              '''<tr><th>Headline:</th><td><input type="text" name="headline" maxlength="50" required /></td></tr>
  <tr><th>Slug:</th><td><input type="text" name="slug" maxlength="50" required /></td></tr>
  <tr><th>Pub date:</th><td><input type="text" name="pub_date" required /></td></tr>
  <tr><th>Writer:</th><td><select name="writer" required>
  <option value="" selected>---------</option>
 
</s> remove             six.text_type(f),
 </s> add             str(f), </s> remove             six.text_type(CategoryForm()),
 </s> add             str(CategoryForm()), </s> remove             six.text_type(f),
 </s> add             str(f), </s> remove             six.text_type(CustomFieldForExclusionForm()),
 </s> add             str(CustomFieldForExclusionForm()), </s> remove         self.assertEqual(six.text_type(instance), 'Bob Woodward is 65')
 </s> add         self.assertEqual(str(instance), 'Bob Woodward is 65')
            str(f),
          # ManyToManyFields are represented by a MultipleChoiceField, ForeignKeys and any
          # fields with the 'choices' attribute are represented by a ChoiceField.
          f = ArticleForm(auto_id=False)
          self.assertHTMLEqual(
            str(f),
              '''<tr><th>Headline:</th><td><input type="text" name="headline" maxlength="50" required /></td></tr>
  <tr><th>Slug:</th><td><input type="text" name="slug" maxlength="50" required /></td></tr>
  <tr><th>Pub date:</th><td><input type="text" name="pub_date" required /></td></tr>
  <tr><th>Writer:</th><td><select name="writer" required>
  <option value="" selected>---------</option>
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