Returns properties XML element.

def _properties_element(self, parent_element):
        """Returns properties XML element."""
        requirements_properties = etree.SubElement(parent_element, "properties")

        req_properties_conf = self.config.get("requirements_import_properties") or {}
        for name, value in sorted(six.iteritems(req_properties_conf)):
            if name == "lookup-method":
                lookup_prop = str(value).lower()
                if lookup_prop not in ("id", "name"):
                    raise Dump2PolarionException(
                        "Invalid value '{}' for the 'lookup-method' property".format(str(value))
                    )
                self._lookup_prop = lookup_prop
            else:
                etree.SubElement(
                    requirements_properties, "property", {"name": name, "value": str(value)}
                )

        return requirements_properties