fix for set_icon crashing with PyObjC 2.2
fix for set_icon crashing with PyObjC 2.2

adamlwgriffiths/Pyglet,niklaskorz/pyglet,adamlwgriffiths/Pyglet,niklaskorz/pyglet,niklaskorz/pyglet,adamlwgriffiths/Pyglet,adamlwgriffiths/Pyglet,niklaskorz/pyglet