To my surprise, "python -m test test_idle" gives a single flash in Windows.  #22770 modified /Lib/test/support/__init__.py and added the creation and packing of a Label.  Those two lines are gone in repository 3.6 but the flash is still there, I added root.withdraw() at line 469 and the flash is gone.

        try:
            from tkinter import Tk
            root = Tk()
            root.withdraw()
            root.update()
            root.destroy()

I added a Label packing back and the flash is still gone here.

> "use test.support.import_module with no reason " ???
I used import_module in test_idle because I was told to and had to to prevent inappropriate test failures.  Please explain what you mean.