specify versions of python for aiida

[ltalirz]

specify exactly which versions of python aiida is supposed to
run on (see p.6 of presentation by @dev-zero on python3
compatibility in aiida from Sep 13th 2018)

[ltalirz]

@sphuber pointed out that aiida does actually not run on python 2.7.6 (released Nov 2013) and lower.
Unfortunately, it isn't possible to specify the python version using classifiers to that precision: https://pypi.org/classifiers/

[coveralls]

Coverage decreased (-0.003%) to 69.014% when pulling b10fdbe on ltalirz:restrict-classifiers into df42835 on aiidateam:provenance_redesign.