chore(dependencies): update futures to 0.3.1

src/client/pool.rs

@@ -733,9 +733,6 @@ impl<T: Poolable + 'static> Future for IdleTask<T> {
     type Output = ();
 
     fn poll(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
-        // Interval is a Stream
-        use futures_core::Stream;
-
         loop {
             match Pin::new(&mut self.pool_drop_notifier).poll(cx) {
                 Poll::Ready(Ok(n)) => match n {},