Free-threading support

[FFY00]

WIP, don't merge.

[FFY00]

The kwlist const change is not strictly required given how it is currently being used, but it's a good idea to avoid static variables when possible. This way the compiler will error out on bad changes, instead of potentially introducing bugs on free-threading builds.

The rest of the static variables are only used to construct str objects that shouldn't change, so that shouldn't introduce issues on free-threading builds. I don't think it's worth dropping the static modifier, as that would introduce performance hits, though very minor.

[FFY00]

From my review of the code, I didn't catch anything else that would be problematic on free-threaded builds — there's no global state other than the static variables mentioned above, and I didn't catch any other instances of unsafe borrowed reference usage.

I'll add some multi-threaded tests now.

[GrahamDumpleton]

Instead of this version check, under free threading is it instead safe to use:

    // dict = PyModule_GetDict(module);
    // round = PyDict_GetItemString(dict, "round");

    round = PyObject_GetAttrString(builtins, "round");

That way same code across all versions.

[GrahamDumpleton]

FWIW. There was a bug in the __round__ hook implementation anyway as it didn't accept ndigits argument. I have fixed that and changed to using PyObject_GetAttrString on assumption that is okay. Because have also made other changes for static vs const, then the only change required for PR may well just be adding:

     #ifdef Py_GIL_DISABLED
         PyUnstable_Module_SetGIL(module, Py_MOD_GIL_NOT_USED);
     #endif

Can you rebase your changes against head of develop branch and check again.

[FFY00]

ACK.

I will rebase.

[GrahamDumpleton]

FWIW. Changing this to const instead of static will result in the compiler complaining (but not erroring), with:

src/wrapt/_wrappers.c:2316:45: warning: passing 'char *const[8]' to parameter of type 'char **' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
 2316 |             "OOO|OOOO:FunctionWrapperBase", kwlist, &wrapped, &instance,

since Python doesn't use const in the prototype of PyArg_ParseTupleAndKeywords and similar functions.

[FFY00]

Humm, I didn't get that warning locally.

[FFY00]

I think everything that was covered in this PR has now been addressed in develop.

[FFY00]

I went ahead and opened Quansight-Labs/free-threaded-compatibility#89, to add wrapt to https://py-free-threading.github.io/tracking/.