Paul, also check to make sure you have installed the latest ActiveTcl Tcl/Tk 8.5.x (not 8.6.x) as described here:  https://www.python.org/download/mac/tcltk/.  One way to tell exactly which minor release of Tcl/Tk (the "x" in "8.5.x") is in use is to click on the "About IDLE" menu item in the "IDLE" menu or, depending on how you launched IDLE, "About Python" in the "Python" menu.  It should be 8.5.18 or higher and not 8.5.9 (the Apple-supplied version).