remove remaining six crumbs

[a-detiste]

Hi,

isort don't need six anymore

https://wiki.debian.org/Python3-six-removal

[sirosen]

Hi, thanks for the contribution! It's always hard to keep unwanted cruft out of configs, so I appreciate help trimming it down.

There's a CI failure, but it is not related to the change. I'm going to merge and come back to fix CI later.

[a-detiste]

Thanks ! I see some project are afraid to remove six because some of their reverse dependencies might need it :-)
There's a lot left to untangle.