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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 async def on_shutdown(self):
 """ Cleans up any outstanding subscriptions. """

        await self._unregister_subscriptions()

        self._accepting = False

        for (ws, _) in self._subscribers:
            await ws.close(code=aiohttp.WSCloseCode.GOING_AWAY,
                           message='Server shutdown')