Great.  I suggest then that you start with untested, normal, non-GUI (non-tkinter) code, assuming that you can find some.  This can mean segregating functional code from gui code if they are currently intertwined.  As point 1 above suggested, IDLE, which dates from about 2000, was originally written without automated testing in mind.

I should have added above
5. 7 months ago, finished #33855 'Minimally test every implementation module'.
In a few cases, that meant import the file, create an instance of the main module, and make a couple of minimal assertions.  The 'coverage' of such files mostly means no syntax errors and X% ran without an exception.

Such code still needs 'does the right thing' tests.  Part of my intention was to make adding those easier by removing the initial boilerplate as a barrier.