filenames generated are easily predictable, it is possible to trick an
unsuspecting program into overwriting another file by creating a
symbolic link with the predicted name.  Fix this by using the
low-level os.open() function with the O_EXCL flag and mode 0700.  On
non-Unix platforms, presumably there are no symbolic links so the
problem doesn't exist.  The explicit test for Unix (posix, actually)
makes it possible to change the non-Unix logic to work without a
try-except clause.

The mktemp() file is as unsafe as ever.