[ndgrigorian]

This PR fixes the warning emitted by test_c_headers on free-threaded builds of CPython, where loading the _c_ext test extension re-enabled the GIL for the rest of the test session

RuntimeWarning: The global interpreter lock (GIL) has been enabled to load module '_c_ext', which has not declared that it can run safely without the GIL.

_c_ext now uses multi-phase initialization and declares Py_mod_gil as Py_MOD_GIL_NOT_USED, as the py_sycl_ls C example already does, and the fixture loading it turns the RuntimeWarning into an error so that this does not regress

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[coveralls]

coverage: 74.963%. remained the same — free-threaded-c-ext-test into master

[github-actions]

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