[3.12] Fix PyVectorcall_Function doc versionadded (GH-107140) (#107173)

Fix PyVectorcall_Function doc versionadded (GH-107140)

The documentation implies that PyVectorcall_Function() was available in Python 3.8.
This is half-true - it was available under a different name. I think it's clearer to set
the "version added" to 3.9.
(cherry picked from commit 0a9b339363a59be1249189c767ed6f46fd71e1c7)

Co-authored-by: da-woods <dw-git@d-woods.co.uk>