Drop support of old python versions

[lopagela]

While browsing the code, I realized that some code is written with python2 in mind (especially the test).
python2 is not supported since January 2020 (https://www.python.org/doc/sunset-python-2/).

I'd also suggest changing the packaging of this library to drop the support of python3.5 (EOL 2020-09-13) and python3.6 (EOL 2021-12-23) (source: https://endoflife.date/python)

Some guidance can be found here: https://packaging.python.org/en/latest/guides/dropping-older-python-versions/

[louis-jaris]

This issue can be closed, the PR was merged