[docs] [issue30456] 2to3 docs: example of fix for duplicates in second argument of isinstance has superfluous parentheses

[Berker Peksag]

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


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stage:  -> backport needed
type:  -> behavior
versions: +Python 3.5, Python 3.6, Python 3.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30456>
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