py3: call codecs.escape_encode() directly
string_escape doesn't exist on Python 3, but fortunately the undocumented
codecs.escape_encode() function exists on CPython 2.6, 2.7, 3.5 and PyPy 5.6.
So let's use it for now.
http://stackoverflow.com/a/23151714
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors