release GIL for detached smol::Executor

[ertosns]

allow_threads doesn't release gil for detached smol::Executor in background, for example the following code block only run in the main thread, and the gil doesn't seem to be released.

async fn executor_run() {
    let ex = Arc::new(smol::Executor::new());
    let fut = ex.spawn(async {
        println!("executor run\n");
    });
    Python::with_gil(|gil| {
            gil.allow_threads(|| {
                fut.detach();
            })
    })
}