I have been in the trenches of Python 2->3 for quite some time now with rpy2, and some of the earlier Python 3 releases did not appear to gain as much traction as the later ones. For rpy2, the target is currently Python 3.5, with the possibility (but absence of support) for going back up to Python 3.3.

Would it make sense to make Python 3 version required on the Gantt chart on your web page ?