Deprecate python versions <3.6

[farrajota]

Since python 2.7 EOL is getting close, and because python 3.6+ contains some neat features (like static typing) that would be nice to have, it has been decided to deprecate any code relevant to python 2.7 and python <3.6.

At this point, I assume that most researchers are using python3 and it won't be much of a big deal maintaining python2 code. This will allow to keep the code in better a shape and add new functionality more easily.