I don't think we can merge this until we require Pillow 11.2.1 (the first version to support avif).
More generally, I also wonder whether it makes sense to add support in such a piecemeal fashion, or whether we should have a more general "forward-all-these-formats-to-pillow" mechanism... (does pillow allow listing the formats it supports?)

Well, the formats are not changing that often, so that piecemeal can be good enough. The problem with the version exists, but is bearable. People will get an error message if their pillow is too old. Maybe check if that message is good enough or whether we should provide a better message.

does pillow allow listing the formats it supports?

From a quick look, I did not see anything in the documentation, but AI suggests

# Get the list of supported formats for opening images
supported_open_formats = Image.OPEN.keys()

# Get the list of supported formats for saving images
supported_save_formats = Image.SAVE.keys()

would need investigation what exactly is in there and whether we can use it.

Edit: Actually, this can be done via PIL.features

Please add a test test_pil_kwargs_avif() similar to the existing test_pil_kwargs_* tests here

Note: Use the feature query like here to selectively skip the test

Ok so I added the test for transparency too, and the kwargs test is basically a copy of the webp test that checks compression quality and kwargs, tell me if there is anything else that should be tested in there.