[issue9253] argparse: optional subparsers

[paul j3]

paul j3 added the comment:

http://stackoverflow.com/questions/22990977/why-does-this-argparse-code-behave-differently-between-python-2-and-3

is answered by this change in how `required` arguments are tested, and how subparsers fell through the cracks.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue9253>
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