Use a closure instead of a labelled block for MSRV compatibility.

src/marker.rs

@@ -365,10 +365,10 @@ impl<'py> Python<'py> {
 
         static THREADS: Mutex<Vec<SyncSender<Task>>> = const_mutex(Vec::new());
 
-        let task_sender = 'dispatch: {
+        let dispatch = move || {
             while let Some(task_sender) = THREADS.lock().pop() {
                 match task_sender.send(task) {
-                    Ok(()) => break 'dispatch task_sender,
+                    Ok(()) => return task_sender,
                     Err(SendError(same_task)) => task = same_task,
                 }
             }
@@ -392,6 +392,8 @@ impl<'py> Python<'py> {
             task_sender
         };
 
+        let task_sender = dispatch();
+
         // 3. Wait for completion and check result
         let result = result_receiver
             .recv()