gh-159517: Do not use ::tk::unsupported::MacWindowStyle in IDLE on Tk 9.1 (GH-159527)

Tk 9.1 no longer provides this command.  Use "wm attributes -stylemask
docmodal" instead, as Tk itself does for its balloon window.  The calls
were silently ignored, so the completion list and the call tip could
grab the focus on macOS.