async functions are not tagged with async

[terraputix]

PyO3 has experimental support for async/await via the experimental-async feature flag.

An async function like the following

use std::{thread, time::Duration};
use futures::channel::oneshot;
use pyo3::prelude::*;

#[gen_stub_pyfunction]
#[pyfunction]
#[pyo3(signature=(seconds, result=None))]
async fn sleep(seconds: f64, result: Option<PyObject>) -> Option<PyObject> {
    let (tx, rx) = oneshot::channel();
    thread::spawn(move || {
        thread::sleep(Duration::from_secs_f64(seconds));
        tx.send(()).unwrap();
    });
    rx.await.unwrap();
    result
}

will yield a stub function

def sleep(seconds:builtins.float, result:typing.Optional[typing.Any]=None) -> typing.Optional[typing.Any]: ...

instead of

async def sleep(seconds:builtins.float, result:typing.Optional[typing.Any]=None) -> typing.Optional[typing.Any]: ...

[kcking]

I implemented a basic version of this here #108, though it has gone stale

[krokosik]

@kcking @termoshtt what is the issue with the PR? Since it does not change any existing logic and deals with a feature of PyO3 itself is there an obstacle to merging it?