gh-155561: Use PySlot API in Modules/_testlimitedcapi.c

[vstinner]

• Issue: C API: Enable limited C API tests on Free Threading #155561

[vstinner]

I hesitated to keep PyMethodDef and PyModuleDef code path when the Py_GIL_DISABLED macro is not defined, but this API is already tested by Modules/xxlimited_3_13.c. IMO it's better and simpler to use PySlot in both case (if Py_GIL_DISABLED is defined or not).

cc @encukou

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Oh. The build fails on WASI because Modules/config.c refers to PyModExport__testlimitedcapi() which doesn't exist.

wasm-ld: error: Modules/config.o: undefined symbol: PyInit__testlimitedcapi

and

error: undefined symbol: PyInit__testlimitedcapi (referenced by root reference (e.g. compiled C/C++ code))

Modules/config.c is generated Modules/config.c.in by Modules/makesetup.

I tried adding PyInit__testlimitedcapi() by calling PyModule_FromSlotsAndSpec() with NULL spec, but this function does crash. The spec argument must be non-NULL.

// Provide a PyInit function for Modules/config.c when _testlimitedcapi 
// extension module is built as a built-in module (ex: on WASI and Emscripten).
PyMODINIT_FUNC      
PyInit__testlimitedcapi(void)
{      
    return PyModule_FromSlotsAndSpec(_testlimitedcapimodule_slots, NULL);
}