Get the base64-encoded signature itself.
        Can only be called once.

def finalize(self):
        """
        Get the base64-encoded signature itself.
        Can only be called once.
        """
        signature = self.signer.finalize()
        sig_r, sig_s = decode_dss_signature(signature)
        sig_b64 = encode_signature(sig_r, sig_s)
        return sig_b64