Returns top XML element.

def _top_element(self):
        """Returns top XML element."""
        attrs = {"project-id": self.config["polarion-project-id"]}
        document_relative_path = self.config.get("requirements-document-relative-path")
        if document_relative_path:
            attrs["document-relative-path"] = document_relative_path
        top = etree.Element("requirements", attrs)
        return top