Change from constants to enums

[melonmouse]

Feature request

Example set of constants that would fit into an enum really well:

https://pillow.readthedocs.io/en/stable/_modules/PIL/Image.html#Image.transpose

That could be refactored to:

class TransposeType(enum.IntEnum):
    FLIP_LEFT_RIGHT = 0
    FLIP_TOP_BOTTOM = 1
    [etc]

FLIP_LEFT_RIGHT = TransposeType.FLIP_LEFT_RIGHT
FLIP_TOP_BOTTOM = TransposeType.FLIP_TOP_BOTTOM
[etc]

Advantages:

• easier to check if a value is as expected (check type of enum value)

Disadvantages:

• have to research if any existing usages can break

Smaller factors:

• (some) refactoring required
• (negligible) performance overhead

[kero-ly]

I think it's designed for casting int value from python to cpp conveniently, and validate it cpp :D

[radarhere]

I've created #5954 to resolve this. See what you think.

[melonmouse]

Awesome work radarhere, left a review there!

[radarhere]

Be aware that #6684 has been proposed, which would revert the changes for this.