Bug#954144: key-mon: Relatively high CPU use on idle machine

[Lawrence D'Oliveiro]

And it turns out the original author is still maintaining the code
(with some help from others). Here
<https://github.com/scottkirkwood/key-mon> is a more up-to-date
version, including conversions to Python 3 and GTK 3, done just
recently. Time to update the Debian package to this new upstream?