feat(hooks): declare native extension free-thread safe (#120)

Declare the dist_instrument_hooks module as supporting the free-threaded
build by calling PyUnstable_Module_SetGIL(Py_MOD_GIL_NOT_USED) under
Py_GIL_DISABLED. Without this, importing the extension on a free-threaded
interpreter re-enables the GIL and emits a RuntimeWarning. The underlying
instrument-hooks C library uses its own internal locking, so the module
is safe to run without the GIL.

Co-authored-by: Claude <noreply@anthropic.com>

Author: art049

src/pytest_codspeed/instruments/hooks/instrument_hooks_module.c

@@ -297,5 +297,9 @@ PyMODINIT_FUNC PyInit_dist_instrument_hooks(void) {
     PyModule_AddIntConstant(module, "MARKER_TYPE_BENCHMARK_START", MARKER_TYPE_BENCHMARK_START);
     PyModule_AddIntConstant(module, "MARKER_TYPE_BENCHMARK_END", MARKER_TYPE_BENCHMARK_END);
 
+#ifdef Py_GIL_DISABLED
+    PyUnstable_Module_SetGIL(module, Py_MOD_GIL_NOT_USED);
+#endif
+
     return module;
 }