Unicode support may be a small thing for scientists, who are usually dealing with numbers rather than strings. But it's absolutely huge for the rest of us, and that alone will drive the big libraries to python3. It'll be slow going - I've essentially stopped work on my biggest personal project as I'm stuck in a mess of libraries that have moved to python3 and libraries that haven't - but unlike with perl6, I haven't seen any major libraries explicitly planning not to support python3.

So yeah, 2018 may be a lower bound for the point at which the scientific community moves to python3. But that doesn't mean it won't happen.