Add `python_requires` to setup

[clemisch]

Please add the required Python version to the setup, such that

python3 -m pip install fabio

on Python 3.5 installs fabio==0.10.2 instead of the newest version. Because of dropped 3.5 support, newest fabio can't be imported.

(I know that I can specify the version to install with pip, but including the required Python version directly in the package is easier for the end-user / maintainer of research servers ;-) )

[kif]

Thanks for the advice. I will try to ensure the proper versions are used. The packaging system will change for the next release of fabio, probably going away from setup.py since we use some features from numpy which are now deprecated.

[kif]

setup.py is gone