I'm interested in helping; getting python3 cemented in as many places as possible is on my list of stuff that needs doing.
Just recently landed a bunch of fixes in Customizer [1], since it had a python rewrite.
Their code is now something like 99% python3 compliant and runs under python2.7 and 3.4 with the same codebase.
There's still one or two problems that py3 throws, but once I track the issue down, it'll probably end up to be a one or two line fix.
The biggest issue with python2/3 on the same codebase is mostly in how the programmer utilizes the available constructs -- It's encouraged to use the
u'I am a {} unicode {} string'.format("beautifully", "formatted") construct, which was missing from python 3.0, 3.1, and 3.2.
So far I've not noticed any issues with 3.4/3.5 compatibility while using 15.10, other than the slight annoyance that python3-dev is still pointed at 3.4, but python3-pip wants to use python3.5-dev for building C extensions (like scrypt or uwsgi).