Not compatible with select/select_biased macro

[kriomant]

use futures::select_biased;
use futures_async_stream::try_stream;

#[try_stream(ok=(), error=())]
pub async fn events(&mut self) {
    select_biased! {
         _ = futures::future::ready(()) => {}
    }
} 

gives

error[E0728]: `await` is only allowed inside `async` functions and blocks
  --> src/rotary.rs:29:13
   |
25 |       #[try_stream(ok=(), error=())]
   |       ---------------------------------------------- this is not `async`
...
29 | /             select_biased! {
30 | |                 _ = futures::future::ready(()) => {}
31 | |             }
   | |_____________^ only allowed inside `async` functions and blocks
   |
   = note: this error originates in the macro `$crate::select_biased_internal` which comes from the expansion of the macro `select_biased` (in Nightly builds, run with -Z macro-backtrace for more info)

[taiki-e]

This is a part of #4.