[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 0a9b339)

Co-authored-by: da-woods <[email protected]>

Author: miss-islington

Doc/c-api/call.rst

@@ -165,7 +165,7 @@ Vectorcall Support API
    This is mostly useful to check whether or not *op* supports vectorcall,
    which can be done by checking ``PyVectorcall_Function(op) != NULL``.
 
-   .. versionadded:: 3.8
+   .. versionadded:: 3.9
 
 .. c:function:: PyObject* PyVectorcall_Call(PyObject *callable, PyObject *tuple, PyObject *dict)