Test cryptography 3.0 on Python 3

[pquentin]

cryptography 3.0 does support Python 2, but issues a deprecation warning at import that I wasn't able to ignore: pyca/cryptography#5346.

[webknjaz]

Ref: pyca/cryptography#5346 (comment).

[pquentin]

Thanks, your comment there fixed it. I guess that's an easier option as it will take care of the Windows issue too.

[webknjaz]

Give me a few min and I'll send a pytest config that would be a better place for all those options...

[webknjaz]

This is what I usually have in other projects: #220. (not setup.cfg because flat is better than nested and namespaces are honking great idea -- it's easier to spot vs having to guess in what file it is, among setup.cfg, tox, pyproject etc)

[webknjaz]

I've also added #221 as a follow-up for #220. You should probably merge them before proceeding here.