the "setuptools+GNU stow" hack doesn't work on zope.interface

[zooko]

The trick of

    ${PYTHON:-python} ./setup.py install --single-version-externally-managed --record=`${CYGPATH} ${PKG_DIR}/${PKG_NAME}-install.log` --prefix=`${CYGPATH} ${PKG_DIR}`

Which can be used to install setuptools packages in a GNUstow-compatible way doesn't work with zope.interface. I don't know why the zope.interface module is not loadable afterward. We don't actually rely on this trick officially -- it isn't described in much detail in the README, and the web page describing it: SetuptoolsAndGNUStow -- isn't complete yet.