Fix variable name in test_threadpool.py

Author: nicoddemus

testing/test_threadpool.py

@@ -164,7 +164,7 @@ def wait_then_put() -> None:
 
     pool.execmodel.start(wait_then_put)
     assert pool.waitall()
-    out, err = capfd.readouterr()
+    _out, err = capfd.readouterr()
     assert err == ""