gh-79644: Fix create_autospec signature handling for functools partial and partialmethod

[claudiubelu]

Fixes both cases reported in gh-79644 (#79644):

1. functools.partial: _get_signature_object() fell through to inspecting func.__call__ for anything that wasn't a plain function / method. For a functools.partial, that resolves to the C slot wrapper's own generic (*args, **kwargs) signature rather than the partial's effective, post-partial-application one. inspect.signature() already has first-class support for computing a functools.partial's effective signature directly; skip the func.__call__ detour and let it do so.

2. functools.partialmethod: _must_skip() special-cases staticmethod / classmethod / plain functions to decide whether self should be dropped from an autospecced method's signature, but had no branch for functools.partialmethod. The raw partialmethod object stored in klass.__dict__ matched none of those cases and fell through to the "not a method" branch, always returning False regardless of is_type. The resolved attribute used to build the child mock's signature (obtained via getattr(), which goes through the descriptor protocol) is a plain function whose signature still includes self when accessed unbound at the class level, so without this fix self's position was checked against the partialmethod's own pre-bound argument instead of being skipped.