      def test_model_field_that_returns_none_to_exclude_itself_with_explicit_fields(self):
          self.assertEqual(list(CustomFieldForExclusionForm.base_fields),
                           ['name'])
          self.assertHTMLEqual(
              six.text_type(CustomFieldForExclusionForm()),
              '''<tr><th><label for="id_name">Name:</label></th>
  <td><input id="id_name" type="text" name="name" maxlength="10" required /></td></tr>'''
          )
  
      def test_iterable_model_m2m(self):
 
</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(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(CustomFieldForExclusionForm()),
      def test_model_field_that_returns_none_to_exclude_itself_with_explicit_fields(self):
          self.assertEqual(list(CustomFieldForExclusionForm.base_fields),
                           ['name'])
          self.assertHTMLEqual(
            str(CustomFieldForExclusionForm()),
              '''<tr><th><label for="id_name">Name:</label></th>
  <td><input id="id_name" type="text" name="name" maxlength="10" required /></td></tr>'''
          )
  
      def test_iterable_model_m2m(self):
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