Update code to python3

[adivardi]

Update the code from python2 to 3, solving some minor changes in syntax and imports.

This will allow the code to run on all in all new ROS releases, since Noetic forward.

[awesomebytes]

Thanks for your PR @adivardi

I just took a look at a previous PR, that I would be keen to integrate first. Your PR has a slight issue to be included directly into 'main' given it won't work in melodic. I think I'll make a new branch for noetic onwards with your changes, after merging #3

[adivardi]

Hi @awesomebytes ,
No problems, I saw the other PR as well and it seems like a useful change.

Two branches are fine, it makes sense to keep the Melodic compatibility.
Let me know if any more changes are needed for this PR.