tests/test-gendoc-el.t

templater: explode if we try to emit a str

Without this if branch, we infinitely recurse in _flatten, which is
very confusing. Something in an hgweb template is trying to write out
a string instead of a bytes on Python 3, and this at least makes it
crash politely.

Differential Revision: https://phab.mercurial-scm.org/D1088

#require docutils gettext

  $ $TESTDIR/check-gendoc el
  checking for parse errors