I just issued a PR that disables testing on python2 on the yt-4.0 branch:
This is motivated by a couple incidences of people bending over backwards to support python2. We already agreed that the yt 4.0 release won't support python2, I just haven't gotten around to disabling testing yet. Originally I was thinking of keeping it running to help make backporting easier, but I think that's overkill. As a compromise in my PR I left the linting check on python2.7 running, that will make it so that we don't merge PRs that include python2-specific syntax (e.g. the @ operator or f-strings). I'll eventually remove that once we no longer maintain the 3.x branch in maintenance mode. At that point we can also remove six and all the future imports.
If anyone has an issue with this, now is the time to raise it. I think this will be uncontroversial but I wanted to bring it up here just in case anyone has problems with the idea.