I have not looked at the code yet, but looking at the philosophy of constraints as seen in the manual, I would say that you did a very very good job! Zero dep, support for old python, support for egg like apps,... I'm really amazed because, the current trend is to have zealots that only want to use and support last versions and take whatever shitty dep they can find that ease their life. But in the real pro dev life, it is brick as good as yours that we need!