Update python to major version only in setup.py

[AdamRJensen]

It's common practice to just list the major version of python as the minor version soon get outdated.

[loicdtx]

Thank you for the suggestion @AdamRJensen, I mostly disagree though. Not a big deal, these classifier are 'just' for indexing purpose on pypi, but as the zen of python says, "explicit is better than implicit" 😉

When in doubt I refer to how widely acclaimed packages are handling it; see for instance rasterio and requests

[AdamRJensen]

Fair point, I see that matplotlib and pandas do both, i.e., have both 'just Python 3' and the minor versions listed

[loicdtx]

Yes, that's also what I did in 48a3f48