Make OS detection work with Python 3

[widhalmt]

The systemd compatible way of running hostnamectl and parsing the output works only with Python 2 so far.

[KevinHonka]

Hey there, I would not recommend using hostnamectl as it is not consistent on different systems. The better way would be to use the built in platform library of python. It should be working with python 2 up to 3.8 and on all known platforms where Python runs.