Add the required Python versions to setup.py

[leouieda]

It's best to specify the compatible Python versions in the setup.py using the python_requires argument. This let's pip et al know if your Python version is compatible and avoids installing broken packages.

It will probably be useful if adopting Pathlib which has some features that are 3.6+ only. I don't see many reasons to use 3.5 since most of the scientific stack has moved on to 3.6.

openjournals/joss-reviews#1425

[benmaier]

see also this comment in openjournals/joss-reviews#1425

Installation instructions: Instructions for installation work and are easy to follow, with the listed dependencies in setup.py being installed upon installation. However, these dependencies are in the documentation and do not appear on the Github readme and should be mentioned in both locations for clarity.