> Python/_warnings.c

I'm not sure if it's a good idea to convert the _warnings module to multi-phase init, since it uses a state stored in the interpreter:

/* Given a module object, get its per-module state. */
static WarningsState *
warnings_get_state(void)
{
    PyInterpreterState *interp = _PyInterpreterState_GET();
    if (interp == NULL) {
        PyErr_SetString(PyExc_RuntimeError,
                        "warnings_get_state: could not identify "
                        "current interpreter");
        return NULL;
    }
    return &interp->warnings;
}

For example, two _warnings instance would share the _warnings.filters list. Maybe it's ok, I don't know.