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

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"
1 job for lars-heartbeat in 2 minutes and 3 seconds (queued for 6 seconds)