"The breakup between Python 2 and 3 has been very slow and painful. Python devs know that, and that's why they won't break compatibility in such big and drastic way ever again."
For most of the ones that aren't compatible I've found an alternative or monkey patched a method to support python 3 (I'm looking at supervisor -> circus and flask-session -> monkey patch because the author seems to have abandoned it).
Fork, don't break, cf Pillow (PIL fork) ~ http://python-pillow.github.io/
What are the most commonly used Py2 packages that need to be Py3?