Current source releases of Python do not specify which version of Tk they should be run with; that is largely up to the distributors of Python (including python.org binary installers for Windows and OS X) and the conventions of the platform the instances are running on.  There are versions of current Python 3.4.x and Python 2.7.x running on supported platforms (like OS X) with various flavors of Tk 8.6, 8.5, and even 8.4.  So Python documentation has to be careful to avoid making assumptions about Tk version-specific features.  FWIW, a link to Tk 8.6 differences is provided on the Tcl/Tk 8.6 release page (http://www.tcl.tk/software/tcltk/8.6.html) -> "Changes in Tcl/Tk 8.6" (http://wiki.tcl.tk/21276).