Topic/explicit python env

@nnrcschmdt I've done some small improvements:

• using "virtualenv" instead of "venv" in the Readme, to be consistent with all the other Readmes
• using an explicit Python version (3.8) für vitualenv, because "python3" uses whatever is available
• Updated the "init" script in "run.sh"

If you are fine with them would be great if you could merge. Thanks :)