Fix: stop accepting new process request when the batcher is stoped (#9)

async_batcher/batcher.py

@@ -59,6 +59,8 @@ async def process(self, item: T) -> S:
         Returns:
             S: The result of processing the item.
         """
+        if self._should_stop:
+            raise RuntimeError("Batcher is stopped")
         if self._current_task is None:
             self._current_task = asyncio.get_running_loop().create_task(self.batch_run())
         logging.debug(item)