Adds requirement XML element.

def _requirement_element(self, parent_element, req_data):
        """Adds requirement XML element."""
        req_data = self._transform_result(req_data)
        if not req_data:
            return

        title = req_data.get("title")
        if not title:
            logger.warning("Skipping requirement, title is missing")
            return
        req_id = req_data.get("id")

        if not self._check_lookup_prop(req_id):
            logger.warning(
                "Skipping requirement `%s`, data missing for selected lookup method", title
            )
            return

        attrs, custom_fields = self._classify_data(req_data)
        attrs, custom_fields = self._fill_defaults(attrs, custom_fields)

        # For testing purposes, the order of fields in resulting XML
        # needs to be always the same.
        attrs = OrderedDict(sorted(attrs.items()))
        custom_fields = OrderedDict(sorted(custom_fields.items()))

        requirement = etree.SubElement(parent_element, "requirement", attrs)

        title_el = etree.SubElement(requirement, "title")
        title_el.text = title

        description = req_data.get("description")
        if description:
            description_el = etree.SubElement(requirement, "description")
            description_el.text = description

        self._fill_custom_fields(requirement, custom_fields)