Allow multiprocessing.pool.ThreadPool.__exit__ to accept None (#7376)

stdlib/multiprocessing/pool.pyi

@@ -111,7 +111,9 @@ class Pool:
     def terminate(self) -> None: ...
     def join(self) -> None: ...
     def __enter__(self: Self) -> Self: ...
-    def __exit__(self, exc_type: type[BaseException], exc_val: BaseException, exc_tb: TracebackType) -> None: ...
+    def __exit__(
+        self, exc_type: type[BaseException] | None, exc_val: BaseException | None, exc_tb: TracebackType | None
+    ) -> None: ...
 
 class ThreadPool(Pool):
     def __init__(