To correct one point from the embedded e-mail: while the original source control system is Mercurial, there is a parallel tree on GitHub.
A number of people, including myself, have contributed to Chirp via GitHub, on both master and py3 branches. All changes go both ways, so developers are free to choose which source control system they prefer.
I think one of the main sticking points to a complete Python 3 version of Chirp is the GUI. With the py3 branch, Chirp is moving to wxPython (wxWidgets), and really, nobody has stepped up to do significant work on that. A start has been made for clone-mode radios, but when I looked into it with a live-mode radio some time ago, it seemed to me that there were design ideas in place, but not enough hints to pick those up and run with them - at least not for someone largely unfamiliar with wxPython, like me. There also seemed to be some other design changes to the way the GUI works that are probably not directly related to wxPython.
A decent amount of the py3 codebase does work, though. For example, I got to the point where I can use chirpc (the CLI version) to read and write memories with my live-mode radio. So the core functionality is there, for at least some drivers. (I know a few other folks have worked on drivers for their specific radios too.)
Martin.
KD6YAM