stubtest: ignore `__vectorcalloffset__` (#13416)
Typeshed currently has 6 allowlist entries relating to `__vectorcalloffset__` attributes in 3.10 and 3.11. I don't think there's value in adding any of these to the stub: as far as I can tell, it seems to be an undocumented, CPython-specific implementation detail of the [vectorcall protocol](https://peps.python.org/pep-0590/)
