Add missing type annotation

Author: bjsowa

rosapi/src/rosapi/async_helper.py

@@ -58,7 +58,7 @@ def future_done_callback(arg):
 
 async def async_sleep(node: Node, delay_sec: float):
     """Block the coroutine for a given time."""
-    sleep_future = Future()
+    sleep_future: Future = Future()
 
     def timeout_callback():
         sleep_future.set_result(None)