We need to test all the front ends with Python 3 and get them working
with it (front ends are both not well-covered by the testsuite, and a
place where bytes/string conversions are particularly likely to be
needed).  By front ends I mean the different web interfaces where
doc/installation.txt says "There are five web interfaces to choose from"
(and then lists six, not five), not just what's in the frontends
directory - all six need to be made to work with Python 3 for us to be
able to document Python 3 support as on a par with that for Python 3.

Regarding front end testing, issue 2141835 provides evidence that the
ZRoundup front end has not actually worked with current Zope for ten
years (so work might well be needed, beyond the patch in that issue, to
get it working with Python 2, before anything could be done to get
ZRoundup working with Python 3).

Also regarding front end testing, issue 2550821 (from 2013) provides a
patch reportedly needed with current mod_python.

Issue 2550566 is someone reporting having difficulty getting WSGI
working following the documentation (but without much information about
exactly what they did).

I've now tested and fixed wsgi_handler and roundup.cgi for Python 3.

wsgi_handler was tested in the form given in the documentation (i.e. the
script as given in the documentation, for a standalone server using
wsgi_handler).  There may be other ways of using WSGI, but if they use
the wsgi_handler code I think they are adequately covered.  In
particular, I think this also covers the "CGI-bin for Limited-Access
Hosting" which uses wsgi_handler.

So the only web interfaces that are as yet uncovered by testing with
Python 3 and so likely to have issues there are ZRoundup and mod_python.
 Both of those have open issues (from 2008 and 2013 respectively)
reporting that the Roundup code does not work (with Python 2) with the
current versions of those packages.  Given that they have been broken
with Python 2 for several years, I don't think possible issues with
those interfaces with Python 3 should block making a Roundup release
with Python 3 support, although of course it would be desirable to get
them fixed and working with both Python 2 and Python 3.