Switch to python3

[cbachhuber]

To enable #3 and for all the benefits of python3 over shell, switch to python3.

The only downside of that change is that interested people can't copy-paste commands directly from the setup script anymore. To alleviate this, consider introducing a dry-run mode which only prints the shell commands it would execute.

[cbachhuber]

You could take the idea of a dry-run output even further and put that output into a file easily accessible online from the readme. E.g. "if you don't like running an unknown script, you can manually execute it's steps, see file xxx'.