Tix is an old Tk widget extension set that has long been superseded by the ttk widgets that were added in Tk 8.5, officially released 10 years ago and since superseded by Tk 8.6.  Tix was useful for users of Tk 8.4 but tkinter is no longer built, tested, or supported with 8.4.  Despite that, we have continued to build and ship Tix with the python.org Windows installers.  The last maintenance release of Tix was in 2008.

As of Python 3.6.0 (https://docs.python.org/3.6/library/tkinter.tix.html), we documented tkinter.tix as:

   Deprecated since version 3.6: This Tk extension is unmaintained
   and should not be used in new code. Use tkinter.ttk instead.

For 3.7, I believe it is time to remove both tkinter.tix and the building of Tix in the Windows builds.