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.