[issue45678] `functools.singledispatchmethod` is missing tests (and is buggy in 3.9)

[Alex Waygood]

Change by Alex Waygood <Alex.Waygood at Gmail.com>:


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pull_requests: +27664
pull_request: https://github.com/python/cpython/pull/29412

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45678>
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