PixelArray issues group

[Starbuck5]

@aatle on PixelArray in #3889 (comment)

I agree with the observations.

After inspecting the code and stubs, I see multiple issues with the original code.
See _pxarray_subscript and _pxarray_ass_subscript in pixelarray.c, and see pixelarray.pyi.

1. (typing only: bug) Missing __getitem__ overloads for two-element tuples other than tuple[int, int], as well as one-element tuples
2. (mentioned issue: API) Should not return None from __getitem__ with empty slice (not how slicing should work)
3. (API/bug) __getitem__ with single ellipsis argument returns self reference instead of new copy
4. (API/bug?) None is intended to be same as ellipsis, but None alone cannot be used as index for __getitem__ or __setitem__
5. (typing only: improvement) Both 1D and 2D PixelArrays currently share the same signatures for type checking, which is a mess
6. (bug) Both __getitem__ and __setitem__ for both 1D and 2D mostly fail to gracefully handle an index that is a one-element tuple (e.g. pxarray[("hello",)])

[shaurya-sharma-dev]

Should #3889 be backported to the main-2.x branch for a potential 2.5.9 release?