Blacknet

- SQL: add a default value for notes on attackers
- Drop compability with Python2, ensure compatibility up to Python3.11
- Upgrade the build system to use `pyproject.toml` with setuptools
- Move console scripts to specific targets within the module
- Add typing annotations on the blacknet module
- Add linting using `ruff`, `isort` and `black`

- Fix ping issues when server is not reachable
- Ensure credentials encoding while transmitting to master server
- Add TCP connection keep-alive parameters

- Add ping/pong for link checker and automatic reconnect
- Few enhancements on thread safety
- Scrubber bugfixes

- Harden communications between master and sensors
- Fix log verbosity on failed connections
- Add systemd example service scripts
- Fix updater script for python3
- Harden scrubber for "corrupted" databases

- Work around hung threads
- Fix thread unsafety on database connections
- Add a limit in authentication retries

- Rename main server to master
- Rename ssh server to sensor
- Fix sys module import in sensor