Sign a bytes-like object using the signing (private) key.

        :param bytes data: The data to sign
        :return: The signed data
        :rtype: bytes

def sign(self, data):
        """Sign a bytes-like object using the signing (private) key.

        :param bytes data: The data to sign
        :return: The signed data
        :rtype: bytes
        """
        if self.signing_key is None:
            raise MissingSigningKeyError("KeyPair does not contain secret key. "
                                         "Use Keypair.from_seed method to create a new keypair with a secret key.")
        return self.signing_key.sign(data)