BUG: avoid AttributeError when setting mode in Pillow v10.1

[FirefoxMetzger]

Closes: #1044

Pillow v10.1 removed the setter for Image.mode as this can have unintended consequences. This is done by making Image.mode a @property and keeping the mode in Image._mode.

While okay in general, it breaks our setup as there is currently no good way of setting 16-bit return types directly (which we want when reading 16-bit grayscale PNGs). As a result, this change breaks our current setup.

This PR introduces a "quick fix" by directly setting Image._mode instead of the previous Image.mode. I'm not fond of this solution, but until something better becomes available this is the only way I can think of to maintain our current behavior.

[FirefoxMetzger]

Looks like there is not just a problem with Image.mode becoming read-only, but also some additions to pillow's GIF reader that cause test failures.

I'll have to set some time aside to figure out where/why this breaks.

[lagru]

@FirefoxMetzger thanks for the quick reaction and working on this!

[FirefoxMetzger]

@lagru I've merged a pin for pillow<10.1 for now (that one passed CI 🙃 ). As for a proper fix, I can pick up this PR again earliest tomorrow evening. However, chances are that I can't fully solve this and will need some feedback from the pillow devs before it can be properly resolved.

[lagru]

No worries. We've applied the same temporary fix to scikit-image and are in no particular hurry. Please don't feel rushed. :)

[FirefoxMetzger]

Looks like I finally managed to solve all the CI issues. Now it's only test coverage left.

I also want to check why we can now use "rawmodes" like "I;16" in PIL.Image.convert, which wasn't possible before but is now. I need to double-check if this internally promotes to 32-bit or reads straight into 16-bit.

[FirefoxMetzger]

For future reference: Recent pillow versions no longer allow direct decoding of 16-bit grayscale PNGs. While we instruct Pillow to do so using the (now available) mode I;16 in Image.convert, this will internally decode the image as 32-bit and then convert to 16-bit in a copy.

I will create a ticket over at pillow and see if we can bring the old functionality back - perhaps in a different way - since it is quite useful and I would eventually like to use it in other places as well.