[issue29383] Reduce temporary unicode object while adding descriptors

[INADA Naoki]

INADA Naoki added the comment:

I think I found better way.

Interned string can be get from descripter. Interning can be reduced
without adding private API.
Please don't review the first patch.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue29383>
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