python 3.x is not yet supported by bitbake. We should work towards it.
I do have a branch with bitbake and OE-Core mostly working with python 3 but it becomes incompatible with python 2 so cannot be merged as is. We therefore have an idea of the problems and work involved now and are making progress towards this but work still remains, updating status accordingly.
http://git.yoctoproject.org/cgit.cgi/poky-contrib/log/?h=rpurdie/python3 was work so far
*** Bug 7622 has been marked as a duplicate of this bug. ***
OE/Bitbake are now using python3 by default: http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=0f2c59367a649de5f57acdccfb4f1fdba9cde730