          self.assertEqual(list(CategoryForm.base_fields),
                           ['description', 'url'])
  
          self.assertHTMLEqual(
              six.text_type(CategoryForm()),
              '''<tr><th><label for="id_description">Description:</label></th>
  <td><input type="text" name="description" id="id_description" required /></td></tr>
  <tr><th><label for="id_url">The URL:</label></th>
  <td><input id="id_url" type="text" name="url" maxlength="40" required /></td></tr>'''
          )
 
</s> remove             six.text_type(CustomFieldForExclusionForm()),
 </s> add             str(CustomFieldForExclusionForm()), </s> remove             six.text_type(f),
 </s> add             str(f), </s> remove             six.text_type(f),
 </s> add             str(f), </s> remove             six.text_type(f),
 </s> add             str(f), </s> remove             six.text_type(f.media),
 </s> add             str(f.media),
            str(CategoryForm()),
          self.assertEqual(list(CategoryForm.base_fields),
                           ['description', 'url'])
  
          self.assertHTMLEqual(
            str(CategoryForm()),
              '''<tr><th><label for="id_description">Description:</label></th>
  <td><input type="text" name="description" id="id_description" required /></td></tr>
  <tr><th><label for="id_url">The URL:</label></th>
  <td><input id="id_url" type="text" name="url" maxlength="40" required /></td></tr>'''
          )
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