You must be in a very luxurious position then to not have to work with any legacy code base. The reality is that there are some giant python 2.7 code bases with forked versions of some major frameworks and libraries. There's no way in hell these places are going to migrate to Python 3 any time in the next decade.
I can't say I've come across an issue caused by an incompatibility between the two since the first couple months after switching...