Thanks, Marc and Ronald, for the analysis.

And thanks for bringing up the issue, Remy. Perhaps you can verify that Ronald's suggestion of importing and launching Tk before calling pyglet or pygame works for you and, if so, follow up with those projects to, if nothing else, document that requirement for their users on macOS (I would guess it wouldn't hurt on other platforms as well but I've guessed wrong before). In any case, I'm closing this issue now.