bpo-36974: rename _FastCallKeywords -> _Vectorcall (GH-13653)

diff --git a/Include/methodobject.h b/Include/methodobject.h
index 5dbe214..e92adde 100644
--- a/Include/methodobject.h
+++ b/Include/methodobject.h
@@ -47,7 +47,7 @@
     Py_ssize_t nargs,
     PyObject *kwargs);
 
-PyAPI_FUNC(PyObject *) _PyCFunction_FastCallKeywords(PyObject *func,
+PyAPI_FUNC(PyObject *) _PyCFunction_Vectorcall(PyObject *func,
     PyObject *const *stack,
     size_t nargsf,
     PyObject *kwnames);