Fix default value from () to []

[PedanticHacker]

The default value of the arrows argument of the chess.svg.board() function can not be an empty tuple, it must be an empty list, since the arrows argument accepts a list of tuples. So, I think, we should fix that.

[PedanticHacker]

It turns out that you can either pass () or [] in the arrows argument to chess.svg.board(), and nothing crashes, but it's more logical for its default value to be [] (i.e., an empty list), since the arrows argument accepts a list of tuples (or a list of Arrow objects) and not just tuples.

[niklasf]

I was trying to avoid the mutable default argument trap, but I think it's fine, because the argument is clearly intended to be read-only.