Receive a signal when waking or suspending?

[Skip Montanaro]

It's not clear there is a straightforward way to catch a signal or get
an event notification when my computer (Dell running XUbuntu 20.04) is
about to sleep or when it's just awakened. The app uses tkinter. Is
there some more-or-less easy way to do this? Mac support would be nice
(I have my eye on a new MacBook one of these days). I suppose bonus
points for something which works on Windows, but that's not a platform
I actually care about.

Thx,

Skip