when trying to update pygtk to 2.10.3 from 2.8.6, it pulls in pyobject 2.12.3, but pygtk < 2.9 is blocking 2.12.3, and it barfs
This is called a blocker ... and for good reason it is blocking, it is not compatible. Simple emerge -C pygtk && emerge pygtk will solve the blocker.
http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?full=1#blocked
*** Bug 158270 has been marked as a duplicate of this bug. ***