[issue20849] add exist_ok to shutil.copytree

[Berker Peksag]

Changes by Berker Peksag <berker.peksag at gmail.com>:


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stage:  -> patch review
versions:  -Python 3.3, Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue20849>
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