This actually turns out to be a bug in pybuild (dh-python). In zope.interface's d/control file we have:
Build-Depends: debhelper (>= 9), dh-python, dpkg-dev (>= 1.16.1~), libpython-all-dbg, libpython-all-dev, libpython3-all-dbg, libpython3-all-dev, python-all-dbg:any, python-all-dev:any, python-setuptools, python-zope.event, python3-all-dbg:any, python3-all-dev:any, python3-setuptools, python3-zope.event Note the ':any' specifier on the interpreter packages. pybuild didn't recognize these. I reported this to Piotr who fixed it in dh-python git, which I tested and verified fixes the problem, so I reassigned this bug to that package.