[docs] [issue30096] Update examples in abc documentation to use abc.ABC

[Éric Araujo]

Éric Araujo added the comment:

> "Best practices" according to whom?

Well at least two senior core developers :)  See the rationale in #16049 when abc.ABC was first added (with Guido approving that inheritance is nicer for most people).

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue30096>
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