Christopher: Add blurb with something like "Make tkinter.Menu.index('none') continue to work in tk 8.7."

@serhiy-storchaka Do we consider tkinter failures due to tcl/tk changes to be bugs, such that fixes should be backported? I am thinking so and plan to merge and backport unless you say something.

I agree that it should be backported.

@chrstphrchvz Do existing tests cover this case? Or a new test is needed?

@chrstphrchvz Do existing tests cover this case? Or a new test is needed?

I did not notice the error in any existing test.test_tkinter tests, so I have added a test for this. The error does prevent IDLE from launching (at least on Aqua), though.

This PR is probably one of the smaller and easier to understand changes needed for Tk 8.7, but in practice it is likely not useful without first addressing e.g.#103194, so presumably there would be more to backport.

Properly testing tkinter, etc, with 8.7 requires a buildbot that installs the latest 8.7, which we don't have yet. I have no idea whether the linux x simulator can simulate 8.7 yet.

Again, please add a blurb. Perhaps "Make tkinter.Menu.index('none') work in tcl/tk versions being developed." is slightly better that what i wrote above.

Thanks@chrstphrchvz for the PR, and@terryjreedy for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11.
🐍🍒⛏🤖

GH-103734 is a backport of this pull request to the3.11 branch.