In debusine-worker: use loop.add_signal_handler instead of generic signal.signal

Carles Pina i Estanyrequested to merge
exit-closing-aiohttp-client-session into devel

loop.add_signal_handler allows to interact with the event loop and debusine-worker can close the aiohttp_client_session correctly.

This avoids a series of messages in the logs each time that debusine-worker was killed (e.g. via systemd). Logs contained:

2022-08-15 11:21:29,650 Unclosed client session
client_session: <aiohttp.client.ClientSession object at 0x7fdf4cff62b0>
2022-08-15 11:21:29,651 Unclosed connector
connections: ['[(<aiohttp.client_proto.ResponseHandler object at 0x7fdf4cf94b80>, 11643.778924467)]']
connector: <aiohttp.connector.TCPConnector object at 0x7fdf4cff62e0>

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