By running the following short program on Linux, you
can see the number of open file descriptors increase:
    import os, pty
    for i in range(10):
        pty.spawn(['true'])
        print len(os.listdir('/proc/%d/fd' % os.getpid()))

This can be fixed by os.close()'ing master_fd.  This
problem seems to exist in CVS head as well as 2.3.

There are also a few changes that would be nice to see
in pty.spawn:

1) get the exit status of the child.  Could be fixed by
adding the following to the end of the function:
    pid, status = os.waitpid(pid, 0)
    return status

2) set master_fd to non-blocking mode, so that the
output is printed to the screen at the speed it is
produced by the child.