Method wait

flow-rs/src/loader/python/utils.rs:61–67

(&mut self, py: Python)

#[pymethods]
impl PyFuture {
    fn wait(&mut self, py: Python) -> PyObject {
        if let Some(chan) = std::mem::take(&mut self.chan) {
            with_context(py, || wait(chan)).unwrap()
        } else {
            py.None()
        }
    }

    fn cancel(&mut self) {
        self.chan = None;