bpo-44254: On Mac, remove disfunctional colors from turtledemo buttons (GH-26448)

On macOS, tk defers to system setting for button background when in normal state.
Give turtledemo button text a color that works on either light or dark background.
(cherry picked from commit af5a3248)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>