[issue18042] Provide enum.unique class decorator

[Nick Coghlan]

Changes by Nick Coghlan <ncoghlan at gmail.com>:


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dependencies: +Code, test, and doc review for PEP-0435 Enum
nosy: +eli.bendersky, ethan.furman

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18042>
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