I tried to push the Python 3 support as good as possible, but currently only the wsgi behavior works. The extras zopeappwsgi and plonetestingz2 need both an unreleased plone.testing, from which it is not clear, whether it fully supports Python3, as it depends on Zope2 to test itself, which leads to the point, where we need a Python 3 version of Zope2.
zopeappwsgi also has a dependency on grok, which is also not Python3 compatible yet.
The following packages should be released before or with the release of this:
z3c.flashmessage (as a dependecy of grokui.admin and grokcore.message)
plone.testing (new release 5.0.1 is needed because of one TypeError already fixed)