gh-143300: implement PyUnstable_SetImmortal

[kumaraditya303]

• Issue: Add an unstable C API for immortalizing objects #143300

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📚 Documentation preview 📚: https://cpython-previews--144543.org.readthedocs.build/

[ZeroIntensity]

I think we also need to document the guarantees regarding which objects may be made immortal. For example, the following results in an assertion failure:

from ctypes import pythonapi, py_object, c_int

set_immortal = pythonapi.PyUnstable_SetImmortal
set_immortal.argtypes = (py_object,)
set_immortal.restype = c_int

print(set_immortal("Hello world"))

[ZeroIntensity]

Looks pretty good.

Bikeshedding: Should this be named PyUnstable_Object_SetImmortal? The Py[Unstable]_ prefix is generally reserved for runtime operations (though it seems PyUnstable_TryIncRef didn't follow that rule).

[kumaraditya303]

Should this be named PyUnstable_Object_SetImmortal?

I'd like to keep it aligned with PyObject_IsImmortal and avoid introducing the Object in between.

[ZeroIntensity]

LGTM!

[vstinner]

Kumar also added PyUnstable_SetImmortal() function for Python 3.13 and 3.14 in pythoncapi-compat: python/pythoncapi-compat#164.