[3.11] gh-103685: Fix tkinter.Menu.index() for Tk 8.7 (GH-103686) (#103734)

gh-103685: Fix tkinter.Menu.index() for Tk 8.7 (GH-103686)

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(cherry picked from commit f0ed293f6aec1c2ed22725301b77d6ccedc2d486)

Co-authored-by: Christopher Chavez <chrischavez@gmx.us>
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>