API/resize: fix failure on greyscale images

[dfdan]

❤️ Thank you for your contribution!

Description

As reported by @fenekku - resize operations were failing on greyscale images. This was caused by the method specified in IIIIF_RESIZE_RESAMPLE being applied to all image regardles of mode.

This setting appears to have been added to force Pillow to perform (higher quality) bicubic resampling on colour images.
Since Pillow 7.0, bicubic is used by default on colour images - thus we can simply bump the Pillow version requirement and remove the override / allow Pillow to select the most appropriate method.

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[fenekku]

Let's add a minor version bump since this does update some dependencies + deprecates some settings.

Thanks!

[fenekku]

This is not needed given the line above, right? arguments will already have a "resample" key set to the value passed as argument to the resize function now that the resample assignment from the setting has been removed.

[dfdan]

Oops, meant to take it out from the line above - I don't wnt to pass "resample=None" to resize.

[fenekku]

Ah I see so something like:

Suggested change

	if resample:
	arguments["resample"] = resample
	if resample is None:
	del arguments["resample"]

to remove resample completely.

[fenekku]

or better yet:

arguments = {...}  # without resample
if resample is not None:
    arguments["resample"] = resample

[fenekku]

I can't merge on this repo. Maybe someone else with merging powers?