Fix compatibility with docutils >= 0.8 under "zope.testrunner --subunit".
When zope.testrunner is asked for subunit output, it replaces sys.stdout with a StringIO. Following http://repo.or.cz/docutils.git/commitdiff/14cde9fb, this confuses docutils into autoclosing the stdout stream in use by subunit, which broke the Launchpad test suite following an upgrade to docutils 0.14.
_PydocParser uses epydoc.markup.restructuredtext._DocumentPseudoWriter, which never writes any output, so the simplest fix for this is just to tell docutils.core.Publisher to use docutils.io.NullOutput for its output, which removes any risk of it going anywhere near stdout.