bpo-14156: Make argparse.FileType work correctly for binary file modes when argument is '-' (GH-13165)

Also made modes containing 'a' or 'x' act the same as a mode containing 'w' when argument is '-'
(so 'a'/'x' return sys.stdout like 'w', and 'ab'/'xb' return sys.stdout.buffer like 'wb').