Exception on ipc file cleanup resulting in socket leak

Running an ipython kernel with `--transport=ipc` and stopping it (via sending an `quit` command though a jupyter-client), generates the following exception:

```
Traceback (most recent call last):
File "/home/amorenoz/.local/share/virtualenvs/insights-cmd-nQG3uYIv/lib/python3.9/site-packages/ipykernel/kernelapp.py", line 242, in cleanup_connection_file
self.cleanup_ipc_files()
File "/home/amorenoz/.local/share/virtualenvs/insights-cmd-nQG3uYIv/lib/python3.9/site-packages/jupyter_client/connect.py", line 426, in cleanup_ipc_files
for port in self.ports:
TypeError: %i format: a number is required, not str
```

The socket files are not cleaned up and leaked

Start in ipykernel/kernelapp.py at cleanup_connection_file and follow its call to cleanup_ipc_files in jupyter_client/connect.py. Reproduce an IPython kernel using --transport=ipc, stop it through jupyter-client, and inspect the port values involved. Done means shutdown completes without the TypeError and the IPC socket files are removed.

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