Supported python versions should be kept in sync

[gdementen]

We no longer test Python3.5 on Travis CI since 099cc4a, but we still announce it is supported in all our projects setup.py and still build conda packages for it (because it is listed in condarecipe/*/conda_build_config.yaml). I guess it is time to really drop Python3.5 support, but next time we stop supporting a version of Python, we should update all these files at the same time to avoid the current situation where we might be creating bogus packages.

[gdementen]

a systemic solution, where we only list supported python versions in one place and the other files "read from there" would be ideal but might be hard to achieve and thus not worth our time

[gdementen]

done for 0.34 but this is a recurrent task