Added enums, deprecate constants

[radarhere]

Resolves #5875, by replacing various sets of constants with IntEnum classes. This would seem to be the exact scenario for which enums were added to Python.

I have retained backwards compatibility by including lines like globals().update(enum.__members__).

I have updated internal usage to use the enums, but not the tests or documentation.

[hugovk]

This is a good way to group things together, but for backwards compatibility we have each one doubled up:

Image.ROTATE_90
Image.Transpose.ROTATE_90
Image.ROTATE_180
Image.Transpose.ROTATE_180
Image.ROTATE_270 
Image.Transpose.ROTATE_270
# etc.

Although dir(Image) only lists the extra Transpose.

Is the idea be that end user code should be updated to use the enum form?

Do we keep the old constants "forever"?

Or do we want to deprecate the old constants and remove them in Pillow 10 (15 months' deprecation period)?

[melonmouse]

To me, this change looks great! I'm a guest in this codebase, so can't judge it completely - thats the only reason I'm commenting and not approving.

One small remark would be that some of the introduced enum classes have names that are a bit generic (like Encoding) and might lead to naming collisions later.

[melonmouse]

To avoid potential naming collisions, what about BlpFormat?

[melonmouse]

To avoid potential naming collisions, what about BlpEncoding?

[melonmouse]

For consistency, what about BlpAlphaEncoding?

[radarhere]

When you talk about naming collisions, I find it hard to imagine that we'd want to add another module-level variable called Format. Or are you thinking of a user running from PIL.BlpImagePlugin import Format?

I was trying to avoid users writing

>>> from PIL import BlpImagePlugin
>>> BlpImagePlugin.BlpFormat.JPEG

Instead,

>>> from PIL import BlpImagePlugin
>>> BlpImagePlugin.Format.JPEG

seems cleaner to me.

[radarhere]

Ok, I've updated the tests, documentation, and deprecated the original constants.

I've also deprecated the "backwards compatibility" constants for Resampling, and deprecated NEAREST for use in Dither, which isn't documented, in favour of NONE.

[hugovk]

Let's also add this to the "Deprecations and removals" page and release notes.

https://pillow.readthedocs.io/en/latest/deprecations.html

[radarhere]

Ok, done. See https://pillow--5954.org.readthedocs.build/en/5954/deprecations.html#constants and https://pillow--5954.org.readthedocs.build/en/5954/releasenotes/9.1.0.html#deprecations

[hugovk]

Thanks!