py3: add extra traceback line present on Python 3.8
I'm not sure why Python 3.8 is outputting this line. It
appears to be a change in behavior of formatting tracebacks on
Python 3.8. So let's add it to expected output.
With this change, test-hook.t now passes on Python 3.8.
Differential Revision: https://phab.mercurial-scm.org/D7946
#require docutils gettext
$ $TESTDIR/check-gendoc da
checking for parse errors