Allow GIL to be disabled in whirlpool C extension

[zmedico]

In 3.13 python extensions need to declare support for GIL features, for example if they don't declare Py_MOD_GIL_NOT_USED then it will cause the GIL to be enabled even when python was launched in free-threaded mode.

This requires "multi-phase initialization" because Py_mod_create is incompatible with m_slots. There's a PyUnstable_Module_SetGIL() function that can be used with single-phase init, but it's an unstable API, so it's better to use multi-phase init. There's no need to use PyModule_GetState() because the whirlpool module has no mutable state.

Bug: https://bugs.gentoo.org/934220

[thesamesam]

Thanks for looking at this already. I haven't dared try -gil yet...

[floppym]

Having just given myself a crash course on "multi-phase initialization", this looks ok to me.

[zmedico]

Having just given myself a crash course on "multi-phase initialization", this looks ok to me.

I've found that there's a PyUnstable_Module_SetGIL() function we can use with single-phase init modules, but it's an unstable API so it seems better to migrate to multi-phase init:

https://docs.python.org/3.13/c-api/module.html#c.PyUnstable_Module_SetGIL

[zmedico]

Thanks for looking at this already. I haven't dared try -gil yet...

I'm running emerge with PYTHON_GIL=0 EPYTHON=python3.13 now, and I haven't noticed any issues. I suspect all of our code is safe, so I don't expect to see an issue unless it is in python itself.