process_one catches wrong QueueEmpty exception type

The `process_one` function [catches](https://github.com/ipython/ipykernel/blob/db5f708f66277110e4336cb4210a12d05323c564/ipykernel/kernelbase.py#L444) `asyncio.QueueEmpty`, but `msg_queue` is a tornado Queue and `get_nowait` [raises](https://github.com/tornadoweb/tornado/blob/aa9d32d5e89b290dad69e0c6655b7497ea492e45/tornado/queues.py#L272) its own [QueueEmpty](https://github.com/tornadoweb/tornado/blob/aa9d32d5e89b290dad69e0c6655b7497ea492e45/tornado/queues.py#L47) type, not the one in asyncio. This looks like it was part of the native async refactor for ipykernel 6, so I don't know if it's as simple as changing what type is caught, or if some other change needs to be made.

ipykernel 6.2, tornado 6.1

Start at ipykernel/kernelbase.py around process_one and compare its caught exception with tornado/queues.py's get_nowait and QueueEmpty definitions linked in the issue. Run the relevant ipykernel tests, and consider the issue done when an empty msg_queue is handled without the wrong exception escaping.

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