DEPEND="dev-python/setuptools net-zope/zope-fixers" zope-interface doesn't need python 3, but zope-fixers does. If zope-fixers can be made to install without it, then great, otherwise, could you use something like DEPEND="dev-python/setuptools || ( net-zope/zope-fixers !dev-lang/python:3.1 )" ?
Fixed.
Reverted and re-opening, as this caused problems (#334443)
zope-fixers are fixes for python3, so they need to be installed if and only if package is installed for python3. only solution that comes to my mind is to add python3 to USE and depend only if it's set. PYTHON_DEPEND supports USE conditionals, so this approach should work.
This bug has been worked around some time ago.