Serialize the value and adds it to the parent.

def serialize_on_parent(
            self,
            parent,  # type: ET.Element
            value,  # type: Any
            state  # type: _ProcessorState
    ):
        # type: (...) -> None
        """Serialize the value and adds it to the parent."""
        dict_value = self._converter.to_dict(value)
        self._dictionary.serialize_on_parent(parent, dict_value, state)