Implement inspect.signature() for functools.cmp_to_key() result

[serhiy-storchaka]

Feature or enhancement

functools.cmp_to_key() returns a custom callable. inspect.signature() returns generic <Signature (*args, **kwargs)> raises a TypeError for the C implementation. The following PR makes it returning the same signature (<Signature (obj)>) for both Python and C implementations (currently it is returned for the Python implementation).

This issue is similar to #118285.

Linked PRs

• gh-118402: Fix inspect.signature() for functools.cmp_to_key() result #118427

[serhiy-storchaka]

Split the TypeError bug in a separate issue #118404.