The heartbreaking thing is that if you're willing to only target 2.7 on the Python 2 front, it's relatively trivial to write in Python 3 and have it be immediately compatible in Python 2 using from __future__ imports.
It all depends on how they wrote their Python 2.7 code. You can write it oblivious to Python 3 and screw things up, or you can write it in a way that you know you'll eventually support Python 3. In the latter case, you might as well just support both right away though.