Move to python3

I believe most distros moved to Python 3 as Python 2 is no longer supported.

It would be great to drop Python 2 support and i.e. fix shebangs to `#!/usr/bin/env python3`.

I know, that it is possible to run this with symlink from python to python3. IMHO it isn't great solution as it makes harder to spot problem with different applications not yet ported to python2.

For legacy systems, it might be easy enough to leave python 2 support in i.e. `python2` branch.