set minimum python version to 3.5

@GregAshton as discussed last year with @ReinhardPrix in #20 (closed), I'd like to

1. set the minimum version to 3.5 right now (which is what I still have on Debian Stretch)
2. make a Zenodo release (will need you to do this)
3. bump again to 3.6 (minimum required by flake8).