Stop raising asyncio.CancelledError if the connection is already closed (#1152)

* Update async_client.py
* Update slack_sdk/socket_mode/async_client.py

Co-authored-by: Kazuhiro Sera <seratch@gmail.com>

Author: mattvonarx

slack_sdk/socket_mode/async_client.py

@@ -122,6 +122,11 @@ async def process_messages(self):
             while not self.closed:
                 try:
                     await self.process_message()
+                except asyncio.CancelledError:
+                    # if self.closed is True, the connection is already closed
+                    # In this case, we can ignore the exception here
+                    if not self.closed:
+                        raise
                 except Exception as e:
                     self.logger.exception(
                         f"Failed to process a message: {e}, session: {session_id}"