pty.fork() does not allocate a controlling tty on
systems that don't have os.forkpty().  The code for
pty.fork() tries to use os.forkpty(), and if it isn't
available, then it tries to do its work in python, but
it doesn't work.

The code does setup stdin, stdout, and stderr to use
the slave device, but it needs to do an explicit open
on the slave device to make it become a controlling tty.

This patch is against pty.py from python 2.3, but the
same change can be made to pty.py from python 2.2 and
it works there.