Make new Pillow antialias constant conditional.
Review Request #12418 — Created June 27, 2022 and submitted
The recent change to move to the modern constants for antialising caused
breakages with older Pillow versions. This change makes it conditional.
Tested with Pillow 9.0.1, which doesn't include the new constant name.
Can we add a comment to each documenting the compatible version range?