I get it now.  [:8] meant .0 to .7 inclusive, like range(8).

I understand that until last spring, the tcl/tk/tkinter situation on Mac was a bit like the old American wild west.  But then Ned more or less tamed it for 3.7 and now 3.6. We no longer install Python to work with system tcl and, I believe, releases compiled against 8.6 would not work with them.

Can you suggest a doc change?

tkinter has a hard-coded requirement for at least 8.4, IDLE for 8.5 (so ttk works).  For future IDLE, that should be at least 8.5.18 or maybe later.  I have thought about adding a startup warning if something older that the expected (tested-against) version is used, but that should be once only, requiring a 'hidden' configuration value.  So maybe not worth the effort.