From http://bugs.python.org/issue14156 
"argparse.FileType for '-' doesn't work for a mode of 'rb'"

I learned that 'stdin/out' can be embedded in an 'open' by using the 'fileno()' and 'closefd=False' (so it isn't closed at the end of open).

With this, the dummy file context that I implemented in the previous patch, could be replaced with:

    partial(open, sys.stdin.fileno(), mode=self._mode,..., closefd=False)

However, as Steven Bethard wrote in the earlier issue, setting up tests when stdin/out will be redirected is not a trivial problem.  So I don't yet have a testable patch.

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And just for the record, the 'osaccess' testing that I wrote earlier, probably should also test that proposed output file string is not already a directory.