Translate project to Python3

As you know, Python2 was used when the project started, and it's reached EOL on 01.01.2020. The translation to Python3 has to be eventually done.

added enhancement refactor labels

I will update the progress here:

Seems like py2to3 translates the source code without too much complaints; however some updates will need to be done to requirements.txt as lxml and cffi are not compatible with python3 on the versions they are currently.

I will introduce the Docker integration in this version, too

Actually, there's a lot more packages that need to be updated; it looks like the resolver for pip is going to change, too; will compensate for that, too

mentioned in merge request !261 (merged)

changed milestone to %1.0.0

closed with merge request !261 (merged)

mentioned in commit c9849a27