tests/test-gendoc-fr.t

tests: handle string escaping/encoding on Python 3

This code was failing on Python 3 for a few reasons:

1) sys.argv is str and str doesn't have a .decode()
2) the "string_escape" encoding was renamed to "unicode_escape"

It is wonky casting to bytes to str to bytes. But this is test
code, so meh. I don't believe we exercise any code paths in these
tests where the arguments aren't ascii.

Differential Revision: https://phab.mercurial-scm.org/D5667

#require docutils gettext

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