I'm still waiting for web2py to shift to python3. From that moment on i'll start working in py3. I tried to work with both versions, but it simply isn't my thing. (Even though Nameko does make it a lot easier) . Still too much hassle and py2 still does the job well.
There used to be a lot of libs not ready for py3 but that's mostly old news. Some bigger frameworks like web2py and zato still cling to py2 but i think users harassed core developers so much they finally gave in.
Checking some of my dependencies I came across aweber_api still stuck at py2, as is the beanstalkc lib and most of all web2py. There are some others like an excel library which hasn't been updated in quite some time, but there will be others available for that.
So once my main framework has migrated i can begin migration a gazillion apps to py3 and see what i encouter. After that it's time to see when everything will go in production. Guess it will take another year or two?