Changes since my #31221 POC:

• Add macros to prevent emitting new compiler warnings: so it's not needed to modify C code using these functions to fix const vs non-const => PEP 670 requires that
• PyUnicode_KIND() return type is now unsigned int, not enum Uncode_Kind
• The following functions return type is now unsigned int instead of int: PyUnicode_IS_COMPACT(), PyUnicode_CHECK_INTERNED(), PyUnicode_IS_READY(), PyUnicode_IS_ASCII(). Previously, the macros just exposed structure members of type unsigned int.
• Don't deprecate PyUnicode_GET_SIZE(), PyUnicode_GET_DATA_SIZE(), PyUnicode_AS_UNICODE(), PyUnicode_AS_DATA()
• Don't add new assertions to ease review.
• Don't convert _PyUnicodeWriter_Prepare() and _PyUnicodeWriter_PrepareKind() macros to functions to keep this PR as small as possible.

I will re-do some on these changes (like deprecate functions) as separated PRs once this PR is merged.

Ah, and I also updated comments :-)

@erlend-aasland @gpshead: Would yo mind to review this PR?

@erlend-aasland: Tell me if you prefer that I split this PR into smaller PRs.

The following functions return type is now unsigned int instead of int: PyUnicode_IS_COMPACT(), PyUnicode_CHECK_INTERNED(), PyUnicode_IS_READY(), PyUnicode_IS_ASCII(). Previously, the macros just exposed structure members of type unsigned int.

Oh. It seems like it's wrong. The following code emits a new compiler warning: comparison of integer expressions of different signedness: ‘unsigned int’ and ‘int’ [-Wsign-compare].

    int skind = PyUnicode_KIND(self);
    ...
    int rkind = skind;
    ...
    assert(PyUnicode_KIND(u) == rkind);

@erlend-aasland: Tell me if you prefer that I split this PR into smaller PRs.

I would prefer that, yes :)

I would prefer that, yes :)

Here is a shorter PR: #91705

I splitted this PR into smaller PRs.