[docs] [issue23156] Update tix install information in tkinter tix chapter of doc

Terry J. Reedy <tjreedy@udel.edu> added the comment: If the obsolete text is not to revised, it should be removed, as Ned suggested. I don't now think a replacement list is needed. Serhiy, if you have any opinion either way, please say go. ---------- stage: patch review -> needs patch versions: +Python 3.8 -Python 2.7, Python 3.4, Python 3.5 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue23156> _______________________________________