It looks like we're still running python2.7 tests on the yt-4.0 branch, at least on travis: https://github.com/yt-project/yt/blob/yt-4.0/.travis.yml
. I'd accept a PR that removed the python2.7 and python3.4 tests (I think next time we merge with master the 3.4 tests will go away, they're already gone on master).
I don't think we've decided the minimum python3 version for the yt 4.0 release. For what it's worth, Python3.5 won't go EOL until five years after its release, which means September 2020, so I'd at least want to have a discussion before dropping support for it out of hand, as users might still be on Python3.5 on LTS distros when we release yt 4.0.