Set the Py_MOD_GIL_NOT_USED flag on modules.

This indicates that the free-threaded build of Python can keep the
GIL disabled when the module is loaded.  Without this module flag,
importing the module will cause the GIL to be re-enabled.  A warning
is emitted if this happens.

src/module.cpp

@@ -40,8 +40,14 @@ BOOST_PYTHON_DECL void scope_setattr_doc(char const* name, object const& x, char
 
 BOOST_PYTHON_DECL PyObject* init_module(PyModuleDef& moduledef, void(*init_function)())
 {
+    PyObject *mod = PyModule_Create(&moduledef);
+#ifdef Py_GIL_DISABLED
+    if (mod != NULL) {
+        PyUnstable_Module_SetGIL(mod, Py_MOD_GIL_NOT_USED);
+    }
+#endif
     return init_module_in_scope(
-        PyModule_Create(&moduledef),
+        mod,
         init_function);
 }