Sign a message using this key.

        :param message: [:class:`bytes`] The data to be signed.
        :param encoder: A class that is used to encode the signed message.
        :rtype: :class:`~SignedMessage`

def sign(self, message, encoder=encoding.RawEncoder):
        """
        Sign a message using this key.

        :param message: [:class:`bytes`] The data to be signed.
        :param encoder: A class that is used to encode the signed message.
        :rtype: :class:`~SignedMessage`
        """
        raw_signed = libnacl.crypto_sign(message, self._signing_key)

        signature = encoder.encode(raw_signed[:libnacl.crypto_sign_BYTES])
        message = encoder.encode(raw_signed[libnacl.crypto_sign_BYTES:])
        signed = encoder.encode(raw_signed)

        return SignedMessage._from_parts(signature, message, signed)