                  field_value = obj_dict["fields"][field_name]
                  # yaml.safe_load will return non-string objects for some
                  # of the fields we are interested in, this ensures that
                  # everything comes back as a string
                  if isinstance(field_value, six.string_types):
                      ret_list.append(field_value)
                  else:
                      ret_list.append(str(field_value))
          return ret_list
  
 
</s> remove                             if hasattr(field_value, '__iter__') and not isinstance(field_value, six.text_type):
 </s> add                             if hasattr(field_value, '__iter__') and not isinstance(field_value, str): </s> remove         self.assertIsInstance(t1.render(c3), six.text_type)
 </s> add         self.assertIsInstance(t1.render(c3), str) </s> remove         if isinstance(self.unique, six.string_types):
 </s> add         if isinstance(self.unique, str): </s> remove     if isinstance(value, six.string_types):  # Handle strings first for performance reasons.
 </s> add     if isinstance(value, str):  # Handle strings first for performance reasons. </s> remove             # Note: We use .decode() here, instead of six.text_type(s, encoding,
 </s> add             # Note: We use .decode() here, instead of str(s, encoding,
                if isinstance(field_value, str):
                  field_value = obj_dict["fields"][field_name]
                  # yaml.safe_load will return non-string objects for some
                  # of the fields we are interested in, this ensures that
                  # everything comes back as a string
                if isinstance(field_value, str):
                      ret_list.append(field_value)
                  else:
                      ret_list.append(str(field_value))
          return ret_list
  
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/serializers/test_yaml.py