[docs] [issue34538] Remove encouragement to author a base class for all Exception subclasses in a module

[INADA Naoki]

INADA Naoki <songofacandy at gmail.com> added the comment:

Thanks, Nathaniel.  I totally concur with you.

This is tutorial section.  We should focus on readers of the tutorial.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34538>
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