[issue35159] Add a link to the devguide in the sidebar of the documentation

[Stéphane Wirtel]

Change by Stéphane Wirtel <stephane at wirtel.be>:


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versions: +Python 2.7, Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35159>
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