The incompatibility arises because zope.session requires zope.i18nmessageid >= 3.4.3 (it won't work with an older one) but completely fails to declare so in its setup.py (there, it says it required zope.i18nmessageid, but without any lower version constraint). So there's actually a bug in zope.session here, which is why I added 'Zope 3' to the affected projects.
The incompatibility arises because zope.session requires zope.i18nmessageid >= 3.4.3 (it won't work with an older one) but completely fails to declare so in its setup.py (there, it says it required zope.i18nmessageid, but without any lower version constraint). So there's actually a bug in zope.session here, which is why I added 'Zope 3' to the affected projects.