arcade.Color can't be unpickled

`arcade.Color` implements a custom `__new__`. According to the `pickle` documentation on [`__getnewargs_ex__()`](https://docs.python.org/3/library/pickle.html#object.__getnewargs_ex__) and [`__getnewargs__()`](https://docs.python.org/3/library/pickle.html#object.__getnewargs__), `arcade.Color` should implement `__getnewargs__()` to enable un/pickling.