I've just (1 hour ago) updated my Python 3 library [1] to support Python 2.7 as well. All I needed to do was pass it through the awkwardduet, the 3to2 fork, and everything just worked.
Writing in Python 3 feels way cleaner though. The objects super calls that didn't need to know their current class was great to use.
The libraries are not that big of a problem, and since you plan to start with it, chances are you probably won't even have a need to use these libraries anyway (you don't have any existing codebase that relies on 2.x or these libraries).
The language is much cleaner in version 3 and from what I read and my personal experience it is much easier to make code written for version 3 work with 2.7 than the other way.