My problem is I'm rarely building something new -- I have various libraries and utilities in python 2, and will extend and use them in my research, along with coauthors in other universities. Therefore, having a "big bang" change to Python 3 is very difficult.

In practice I imagine some day some PhD student will take it upon themselves to rewrite a significant chunk of libraries to be python 3 compatible, and then we could move on from there, but I wouldnt want to force anyone to spend the time doing a massive Python 3 rewrite.

I'm curious whether you've looked at __future__ and libraries like six? I've found it quite easy to write Python 2/3 compatible code with them and a little planning. I like this approach to maintenance of legacy libraries as it at least prevents making the problem worse.