Remove six dependency

[plamut]

Now that we don't have to support Python 2.7 anymore, we can get rid of six.

If the number of cleanup changes turns out to be very high, it is probably better to remove six gradually, in which case this ticket should be treated merely as a reminder.

[plamut]

I checked the source code:

$ git grep 'six.' -- '*.py' | wc -l
108
$ git grep 'from six import' -- '*.py' | wc -l
0

108 usages of six is not all that much, and there are no from six import ... lines, either. This seems pretty manageable IMHO, not too much for a single PR.

Edit: There are also about a dozen or so from six.moves imports that I initially missed, but that's about it.