avoid a reference cycle in subinterpreter pools

Author: graingert

Lib/concurrent/futures/interpreter.py

@@ -205,8 +205,7 @@ def run(self, task):
             assert res is None, res
             assert pickled
             assert exc_wrapper is not None
-            exc = pickle.loads(excdata)
-            raise exc from exc_wrapper
+            raise pickle.loads(excdata) from exc_wrapper
         return pickle.loads(res) if pickled else res