py3: hunt down str(exception) instances and use util.forcebytestr
I decided to grep around for \sstr\( and see what low-hanging fruit
that showed me. This was part of that hunt. That grep pattern still
has some things worth exploring.
Differential Revision: https://phab.mercurial-scm.org/D2440
#require docutils gettext
$ $TESTDIR/check-gendoc zh_CN
checking for parse errors