tests/test-gendoc-zh_CN.t

py3: replace file() with open() in test-convert-mtn.t

While I was here, I added b'' prefixes to make sure we write things correctly in
bytes mode on Python 3.

Differential Revision: https://phab.mercurial-scm.org/D2111

#require docutils gettext

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