gh-129578: Fix collection of doctests from wrapped objects

[viccie30]

Add follow_wrapped parameter to doctest.DocTestFinder.find to
unwrap objects before searching them for doctests.

Add functools.update_wrapper call to constructor of
functools.singledispatchmethod to let it be searched by doctest.

• Issue: Doctest ignores methods decorated with functools.singledispatchmethod #129578

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📚 Documentation preview 📚: https://cpython-previews--129579.org.readthedocs.build/

[picnixz]

Is there a way to do it without applying update_wrapper? I'm not entirely sure that it can be applied in all situations and the omission was not something deliberate. If update_wrapper is needed, we may also have a dedicated issue for that as this may change runtime assumptions for downstream users, so more testing may be needed (independently of doctest itseff).

cc @serhiy-storchaka

[viccie30]

Is there a way to do it without applying update_wrapper? I'm not entirely sure that it can be applied in all situations and the omission was not something deliberate. If update_wrapper is needed, we may also have a dedicated issue for that as this may change runtime assumptions for downstream users, so more testing may be needed (independently of doctest itseff).

It might be possible by letting doctest use an object's __doc__ attribute always and setting that explicitly in the constructor of singledispatchmethod. Right now, doctest only collects docstrings from function-like and class objects.