gh-94473: Flatten arguments in tkinter.Canvas.coords() (GH-98479)

It now accepts not only "x1, y1, x2, y2, ..." and "[x1, y1, x2, y2, ...]",
but also "(x1, y1), (x2, y2), ..." and "[(x1, y1), (x2, y2), ...]".