[issue21767] singledispatch docs should explicitly mention support for abstract base classes

[Batuhan]

Change by Batuhan <batuhanosmantaskaya at gmail.com>:


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keywords: +patch
pull_requests: +16680
stage: needs patch -> patch review
pull_request: https://github.com/python/cpython/pull/17171

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue21767>
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