Un-deprecate getdata(); have it return an ImageLinearAccess object

[akx]

Follows up on #9292 to un-deprecate getdata(), since get_flattened_data() is not a perfect replacement for linear access to image pixels given its upfront memory and time costs. (See #9292 (comment) for rationales.)

Fixes #9261 (since getdata now returns a real, well-typed iterator/access object). Given x.py like

# pyright: strict
from __future__ import annotations
from PIL import Image
im = Image.new("RGB", (4, 4))
printed = list(im.getdata())

pyright fails on main:

(main) $ uvx --quiet --with pytest --with numpy pyright@1.1.411 x.py
  x.py:5:1 - error: Type of "printed" is partially unknown
    Type of "printed" is "list[Unknown]" (reportUnknownVariableType)
  x.py:5:16 - error: Argument of type "ImagingCore" cannot be assigned to parameter "iterable" of type "Iterable[_T@list]" in function "__init__"
    "ImagingCore" is incompatible with protocol "Iterable[_T@list]"
      "__iter__" is not present (reportArgumentType)
2 errors, 0 warnings, 0 informations

but is fine on this branch:

(image-getdata-linear-access) $ uvx --quiet --with pytest --with numpy pyright@1.1.411 x.py
0 errors, 0 warnings, 0 informations

The optimizations from #9881 still help this too and don't conflict here.

[akx]

Didn't think I'd bump into a PyPy bug implementing this, but here we are... pypy/pypy#5526

[radarhere]

Could you identify exactly what workaround you added for that, so that one day when the PyPy fix has propagated, we are able to remove it?

[radarhere]

What do you mean by 'weird'?

[akx]

Weird, as in things that a normal user shouldn't be touching (which was the thing in the first place). I can remove the comment if it's too weird in itself. :D

[akx]

Could you identify exactly what workaround you added for that, so that one day when the PyPy fix has propagated, we are able to remove it?

The workaround is the linear access type also act as a mapping (.tp_as_mapping = &linear_access_as_mapping), since CPython's and PyPy's mechanisms prefer that for getitem when available. When the object does not act like a mapping, CPython and PyPy will use the sequence item accessor (linear_access_item) instead, but before pypy/pypy@e6ed5cc is shipped in PyPy, it doesn't do the negative-index adjustment like CPython does.

IOW, there's nothing that necessarily needs to be removed.