doufo.ConvertDict().sorted_converters_key: sort converter keys  
        sort key according to their relationship (if parent- and child-class)   
            or their hash value.
        Args:  
            `self`

def sorted_converters_keys(self):
        """doufo.ConvertDict().sorted_converters_key: sort converter keys  
        sort key according to their relationship (if parent- and child-class)   
            or their hash value.
        Args:  
            `self`  
        """
        keys = sorted(self.converters.keys(),
                      key=cmp_to_key(tuple_type_compare))
        return {k: self.converters[k] for k in keys}