[issue20197] Support WebP image format detection in imghdr module

[Berker Peksag]

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


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nosy: +berker.peksag
versions: +Python 3.5 -Python 2.7, Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue20197>
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