docs: def _unbytes(bytestr):
      """
      Returns a bytestring from the human-friendly string returned by `_bytes`.
  
      >>> _unbytes('123456')
      '\x12\x34\x56'
      """
      return ''.join(chr(int(bytestr[k:k + 2], 16))
                     for k in range(0, len(bytestr), 2))