Use new grammar for except clauses

Add a style test for new grammar and convert all current instances of old grammar. The new grammar is optional in 2.5 < Python < 3.0 and the new normal in Python 3. Now that we have dropped Py2.5 support, this change will reduce the number of conversions made by 2to3.

Please pull from the exp-new-except branch.

https://gitorious.org/stem-robinson/stem-robinson/commits/exp-new-except

Trac:
Username: robinson

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