[docs] [issue28733] Show how to use mock_open in modules other that __main__

Michał Bultrowicz added the comment: Ok, I've checked again and now patching "file_writer.open" works. I have no idea what I was doing wrong the last time I checked... So I guess I'll close the issue. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue28733> _______________________________________