How to use the josepy.JWS.sign function in josepy

def gen_validation(self, account_key, alg=jose.RS256, **kwargs):
        """Generate validation.

        :param .JWK account_key: Private account key.
        :param .JWA alg:

        :returns: This challenge wrapped in `.JWS`
        :rtype: .JWS

        """
        return jose.JWS.sign(
            payload=self.json_dumps(sort_keys=True).encode('utf-8'),
            key=account_key, alg=alg, **kwargs)