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.