Mark StubHandler.open() as abstract

[radarhere]

Within our stub plugins, we use (1, 1) as a placeholder size.

Pillow/src/PIL/BufrStubImagePlugin.py

Lines 51 to 53 in 627f3ea

	# make something up
	self._mode="F"
	self._size=1, 1

1. I think (0, 0) might more clearly indicate that there's nothing there yet.
2. What if we added a requirement that when the handler is implemented, image size must be set (to something more than (0, 0))? To communicate this better, we could also mark StubHandler.open() as abstract.