Deprecate PyASCIIObject/PyUnicodeObject

[encukou]

Feature or enhancement

Deprecate, and plan removal of, direct access to string internals:

• `PyASCIIObject, PyCompactUnicodeObject, PyUnicodeObject structs
• the PyUnicode_IS_COMPACT macro

This will allow reorganizing the structs for optimizations or new features.

One possible new feature is better support for subclasses that need additional C state. The NumPy and PyObjC projects currently do this. Both can adapt to CPython changes relatively quickly, so we can provide unstable API for their use case.

See discussion.

C API WG vote pending.

Linked PRs

• gh-131510: Deprecate PyUnicodeObject structs, revert memory layout #130697