Free-threaded Python: PYTHON_GIL=0 is ignored under -I/-E; use -X gil=0

Tested on uv's CPython 3.14.10t and 3.15.2t (Linux) with two one-function C extensions built by gcc. Importing a module whose PyModuleDef lacks the {Py_mod_gil, Py_MOD_GIL_NOT_USED} slot turns the GIL back on for the whole process (sys._is_gil_enabled() False → True) and emits a RuntimeWarning located at <frozen importlib._bootstrap>:491, not at your import line. The warning says to run with PYTHON_GIL=0 or -Xgil=0, but PYTHON_GIL=0 has no effect when Python runs with -E or -I (isolated mode ignores PYTHON* variables): the GIL still comes on. -X gil=0 works, including together with -I. To catch the problem in tests, -W error::RuntimeWarning makes the import raise, but the module is already in sys.modules and the GIL stays enabled after the exception, so assert not sys._is_gil_enabled() after imports instead of relying on the warning. Multi-phase init (PyModuleDef_Init with the slot under #ifdef Py_GIL_DISABLED) keeps the GIL off.