Use PEP 604 syntax wherever possible (#7493)

stdlib/pickle.pyi

@@ -189,7 +189,7 @@ if sys.version_info >= (3, 8):
         def __init__(self, buffer: Any) -> None: ...
         def raw(self) -> memoryview: ...
         def release(self) -> None: ...
-    _BufferCallback = Optional[Callable[[PickleBuffer], Any]]
+    _BufferCallback = Callable[[PickleBuffer], Any] | None
     def dump(
         obj: Any,
         file: _WritableFileobj,