Return a signature for the given message.

def sign(self, msg: Dict) -> Dict:
        """
        Return a signature for the given message.
        """
        ser = serialize_msg_for_signing(msg, topLevelKeysToIgnore=[f.SIG.nm])
        bsig = self.naclSigner.signature(ser)
        sig = base58.b58encode(bsig).decode("utf-8")
        return sig