`namedtuple.__replace__` should return `Self`

[max-muoto]

__replace__ was added onto NamedTuple as a part of 3.13 related changes, however, instead of returning None, it should return Self. Looking in cpython we can see this is the case.

Furthermore, this is important for compatibility with copy.replace. In python/typeshed#12262 we're adding a protocol that requires replaceable objects implement:

class _SupportsReplace(Protocol):
    def __replace__(self, *args: Any, **kwargs: Any) -> Self: ...

With namedtuple.__replace__ returning None, users will return into false positives here, when in reality it should work just fine.

[ilevkivskyi]

Do you have an actual test case that fails? Also maybe read a comment on add_method() that says

            ret: Type | None,  # None means use (patched) self-type

[max-muoto]

Do you have an actual test case that fails? Also maybe read a comment on add_method() that says

            ret: Type | None,  # None means use (patched) self-type

Apologies, didn't realize that would happen by default - I think this is fine to close out in that case.