Removed PixelAccess protocol

[radarhere]

Suggestion for python-pillow#8032

I think that after 10.4.0, we'll have no need for the PixelAccess protocol - we can just use core.PixelAccess. If so, then it's not helpful to add a public protocol only to remove it in the next version. This returns core.PixelAccess | PyAccess.PyAccess | None instead.

[nulano]

Thanks. I wasn't sure how we wanted to refer to classes in core and figured a protocol would work, but this seems fine as well. (I was going to update the return type to clarify it instead, but didn't have time.)

This looks fine, except that the core.PixelAccess object only has __getitem__ and __setitem__, not getpixel and putpixel:
https://github.com/python-pillow/Pillow/blob/4b258be3bbedbdd54ef65140a14982b086d10c7b/src/_imaging.c#L3901-L3924

I've pushed a commit with that fixed instead.