fix(heartbeat-monitor): port the "PyHeartBeat" script to Python3

See merge request aura/engine!8

docs: fix spelling issues

See merge request aura/engine!4

Improve graceful shutdown. #88

See merge request aura/engine!10

Thanks, codespell!

In addition to porting it to python3, a number of documentation and code
issues were solved:

* doc: proper order of arguments
* doc: mention debug mode (previously via "__debug__", now "DEBUG=1")
* doc: provide example for manual heartbeat packets
* code: specify timeout for "socket.recvfrom" (avoid hanging)
* code: handle parameters properly (cast to int/float)
* style: format with "black"