[issue23897] Update Python 3 extension module porting guide

[Berker Peksag]

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


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components: +Documentation
stage:  -> needs patch
type:  -> enhancement
versions: +Python 3.5, Python 3.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue23897>
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