I want the simplest possible way to pop up simple dialogs in Python scripts. Ideally, the solution would:
This is not possible. If you want simple then you have to use Tkinter because that is what is included. If Tkinter is not good enough then you will have to choose and package a GUI for each platform separately.
I suggest that you do use Tkinter and wrap the parts that you need in a class that will be even simpler to use.