Vision: Remove six dependency

[fmassa]

Now that we have transitioned to Python3-only, I believe we can remove the uses of six and instead use standard libraries / functions from python3.

We should also remove six from CI and builds.

[pmeier]

I could take that up. As I've never worked with Python2, is there anything that I need to know before I start?

[fmassa]

Thanks @pmeier !

I think it should pretty much just be replacing things like

from six import ...

with the corresponding function in Python3. Let me know if you stumble in any specific issue