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), ...]".

Lib/tkinter/__init__.py

@@ -2817,7 +2817,7 @@ class Canvas(Widget, XView, YView):
 
     def coords(self, *args):
         """Return a list of coordinates for the item given in ARGS."""
-        # XXX Should use _flatten on args
+        args = _flatten(args)
         return [self.tk.getdouble(x) for x in
                            self.tk.splitlist(
                    self.tk.call((self._w, 'coords') + args))]