Hello Patrik
I believe the current maintainer is Ned Harvey who took over in May 2013, but I think his actions were confined to some fixes to the website rather than any work on the program code, and he hasn't been visible here recently. Before him the maintainer was Andrew Ferguson and before him Ben Escoto who wrote the original program.
As you are a python coder and have done some great work on rdiffweb, I am sure Ned as well as all of us who use rdiff-backup would really welcome the contribution you could make to the rdiff-backup code, including making it python3-compatible. If Ned no longer wants to be involved, perhaps you could become the maintainer?