nder/receiver bookeeping. Not threadsafe.

        Connecting & disconnecting leave behind a small amount of bookeeping
        for the receiver and sender values. Typical workloads using Blinker,
        for example in most web apps, Flask, CLI scripts, etc., are not
        adversely affected by this bookkeeping.

        With a long-running Python process performing dynamic signal routing
        with high volume- e.g. connecting to function closures, "senders" are
        all unique object instances, and doing all of this over and over- you
        may see memory usage will grow due to extraneous bookeeping. (An empty
        set() for each stale sender/receiver pair.)

        This method will prune that bookeeping away, with the caveat that such
        pruning is not threadsafe. The risk is that cleanup of a fully
        disconnected receiver/sender pair occurs while another thread is
        connecting that same pair. If you are in the highly dynamic, unique
        receiver/sender situation that has lead you to this method, that
        failure mode is perhaps not a big deal for you.
        N)