Both packages install files to [..]/site-packages/zope/testing. net-zope/zopeinterface-3.4.1 does not install these files any more. I suggest to add a conflict/blocking marker for "<net-zope/zopeinterface-3.4" or so to the net-zope/zopetesting ebuild. Reproducible: Always
The oldest version of net-zope/zopeinterface in the official tree is now 3.5.1. This bug can be closed, closing.