[issue7859] support "with self.assertRaises(SomeException) as exc:" syntax

[Ezio Melotti]

Changes by Ezio Melotti <ezio.melotti at gmail.com>:


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nosy: +ezio.melotti
stage:  -> needs patch

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue7859>
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