There's only PYTHON_COMPAT=( python3_2 ) in the rhythmbox ebuild while there is python-3.3 in unstable, which I have already switched entirely to. I tried adding python3_3 to the rhythmbox ebuild, re-emerged, and it seemed to work just fine - I tested with magnatune plugin which, as I understand, is python-based. Can you have a look at it?
I didn't want to install python-3.2 just to run rhythmbox when I already had python-3.3 installed, so I made the same change as Bartosz, although I added it like this: PYTHON_COMPAT=( python{3_2,3_3} ) It seems to work fine although I haven't tried any of the Python plugins.
I tried to add it but repoman will complain until others (hence the "depend on") are migrated too
This was solved some days ago