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

[Terry J. Reedy]

Terry J. Reedy <tjreedy at 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 at bugs.python.org>
<https://bugs.python.org/issue23156>
_______________________________________