Return the secret_key, converted to bytes and cached.

def secret_key_bytes(self):
        ''' Return the secret_key, converted to bytes and cached.

        '''
        if not hasattr(self, '_secret_key_bytes'):
            key = self.secret_key()
            if key is None:
                self._secret_key_bytes = None
            else:
                self._secret_key_bytes = codecs.encode(key, "utf-8")
        return self._secret_key_bytes