Set up module.

    Open a UDP socket, and listen in a thread.

def _setup():
    """ Set up module.

    Open a UDP socket, and listen in a thread.
    """
    _SOCKET.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
    _SOCKET.bind(('', PORT))
    udp = threading.Thread(target=_listen, daemon=True)
    udp.start()