refactor singleton instance of BECDispatcher

Instead of:

def BECDispatcher():
    global _bec_dispatcher
    if _bec_dispatcher is None:
        parser = argparse.ArgumentParser()
        parser.add_argument("--bec-client", default=None)
        args, _ = parser.parse_known_args()

        _bec_dispatcher = _BECDispatcher(args.bec_client)
    return _bec_dispatcher

to use more idiomatic approach with __new__ as in BECClient .