remove documentation about speciality of setupterm's descriptor, use os.devnull

[jquast]

It's actually perfectly fine to call curses.setupterm(kind, open(os.devnull).fileno()).

More importantly, this removes a chunk of documentation and comments, such as:

            Terminal initialization sequences will be sent to ``stream`` if it
            has a file descriptor and to ``sys.__stdout__`` otherwise. (``setupterm()`` demands to send them
            somewhere, and stdout is probably where the output is ultimately
            headed. If not, stderr is probably bound to the same terminal.)

and

            # (...) Explicit args make setupterm()
            # work even when -s is passed to nosetests. Lean toward sending
            # init sequences to the stream if it has a file descriptor, and
            # send them to stdout as a fallback, since they have to go
            # somewhere.

This issue backports upstream erikrose#95

[jquast]

Although it is fine, I'm having a hard time dealing with the file descriptor leaks, see comments in #146