So mopidy wants python2.7. Ok, that's fine with me after all both versions are installed. /usr/bin/mopidy however starts with #!/usr/bin/python which is typically a symlink to /usr/bin/python3 if both python2.7 and python3 are installed.
So the first fix is to change #!/usr/bin/python to #!/usr/bin/python2 in /usr/bin/mopidy.
Try to run mopidy again - still not working:
Traceback (most recent call last):
File "/usr/bin/mopidy", line 7, in <module>
from mopidy.__main__ import main
ImportError: No module named 'mopidy'.__main__
What's wrong now? Turns out the the packages (mopidy, pykka, certifi, setuptools and tornado) are installed to /usr/lib/python3.4/site-packages by running pip install -U mopidy and are therefore not found by python2.7. To fix this it is necessary to not call pip but pip2 as it will install packages to the library path of python2.7 /usr/lib/python2.7/site-packages. Check this by calling pip -V or pip2 -V which returns something like pip 6.1.1 from /usr/lib/python3.4/site-packages (python 3.4) and pip 6.1.1 from /usr/lib/python2.7/site-packages (python 2.7) respectively.
So the second issue is to pip2 install -U mopidy instead of pip install -U mopidy.
(The latter seems pretty obvious given the fact that mopidy runs with python2.7 but is still a little pitfall one could stumble about.)
Now you should have a mopidy jukebox up and running.