Use _ for unused variables

Co-authored-by: Jacob Tomlinson <jacobtomlinson@users.noreply.github.com>

Author: DimitriPapadopoulos

distributed/tests/test_client.py

@@ -6428,7 +6428,7 @@ async def test_as_completed_async_for_results(c, s, a, b):
     results = []
 
     async def f():
-        async for _future, result in ac:
+        async for _, result in ac:
             results.append(result)
 
     await f()