Return the best selector for the platform

def DefaultSelector(sock):
    "Return the best selector for the platform"
    global _DEFAULT_SELECTOR
    if _DEFAULT_SELECTOR is None:
        if has_selector('poll'):
            _DEFAULT_SELECTOR = PollSelector
        elif hasattr(select, 'select'):
            _DEFAULT_SELECTOR = SelectSelector
        else:
            raise RedisError('Platform does not support any selectors')
    return _DEFAULT_SELECTOR(sock)