bpo-39245: Switch to public API for Vectorcall (GH-18460)

The bulk of this patch was generated automatically with:

    for name in \
        PyObject_Vectorcall \
        Py_TPFLAGS_HAVE_VECTORCALL \
        PyObject_VectorcallMethod \
        PyVectorcall_Function \
        PyObject_CallOneArg \
        PyObject_CallMethodNoArgs \
        PyObject_CallMethodOneArg \
    ;
    do
        echo $name
        git grep -lwz _$name | xargs -0 sed -i "s/\b_$name\b/$name/g"
    done

    old=_PyObject_FastCallDict
    new=PyObject_VectorcallDict
    git grep -lwz $old | xargs -0 sed -i "s/\b$old\b/$new/g"

and then cleaned up:

- Revert changes to in docs & news
- Revert changes to backcompat defines in headers
- Nudge misaligned comments

Modules/_functoolsmodule.c

@@ -213,7 +213,7 @@ partial_vectorcall(partialobject *pto, PyObject *const *args,
 static void
 partial_setvectorcall(partialobject *pto)
 {
-    if (_PyVectorcall_Function(pto->fn) == NULL) {
+    if (PyVectorcall_Function(pto->fn) == NULL) {
         /* Don't use vectorcall if the underlying function doesn't support it */
         pto->vectorcall = NULL;
     }
@@ -440,7 +440,7 @@ static PyTypeObject partial_type = {
     0,                                  /* tp_as_buffer */
     Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
         Py_TPFLAGS_BASETYPE |
-        _Py_TPFLAGS_HAVE_VECTORCALL,    /* tp_flags */
+        Py_TPFLAGS_HAVE_VECTORCALL,     /* tp_flags */
     partial_doc,                        /* tp_doc */
     (traverseproc)partial_traverse,     /* tp_traverse */
     0,                                  /* tp_clear */