tkinter: bind() method type-hint incorrectly has 'add' argument as a bool

[JohnVillalovos]

The documentation for the bind() method https://docs.python.org/3/library/tkinter.html#bindings-and-events shows that the add parameter is a string.

The work from #4347 made add be type-hinted as a bool.

So type-checking fails if doing: add="" or add="+" which is the documented way to use add in the bind() method.