Add python_requires='>=3.5' to setup.py

[vphilippon]

See #205

This strictly enforces the requirements for Python >=3.5.

After the merge+release, still needs a yank of the 0.17.0 release to make the Python2.7 ecosystem "safe" again :) .

[ChristineTChen]

@tobgu can you take a look at this? This is urgent as it affects many people/pipelines, thank you 🤞

[andrewegel]

+1 on yanking and re-issuing a 0.17.0 release.

[tobgu]

Thanks!

[wasilukm]

Please note that this still may not prevent pip from installing incompatible version. In my case pip is trying to install 0.17.1 and quits with following error:
ERROR: Package 'pyrsistent' requires a different Python: 2.7.17 not in '>=3.5'

[AlanRosenthal]

I'm seeing the error when installing packages with conda