Pillow: mark MAX_IMAGE_PIXELS as non-final

[DMRobertson]

I don't think it was correct for #10411 to mark MAX_IMAGE_PIXELS as final. Quoting my reasoning from #10411 (comment):

According to pillow.readthedocs.io/en/stable/reference/Image.html#PIL.Image.open:

This threshold can be changed by setting MAX_IMAGE_PIXELS. It can be disabled by setting Image.MAX_IMAGE_PIXELS = None.

I can't see any mention of this [MAX_IMAGE_PIXELS] in pillow.readthedocs.io/en/stable/releasenotes/10.0.0.html either.

[AlexWaygood]

Thanks!

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉