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

Include/cpython/abstract.h

@@ -67,7 +67,7 @@ PyVectorcall_Function(PyObject *callable)
 {
     assert(callable != NULL);
     PyTypeObject *tp = Py_TYPE(callable);
-    if (!PyType_HasFeature(tp, _Py_TPFLAGS_HAVE_VECTORCALL)) {
+    if (!PyType_HasFeature(tp, Py_TPFLAGS_HAVE_VECTORCALL)) {
         return NULL;
     }
     assert(PyCallable_Check(callable));
@@ -178,7 +178,7 @@ PyAPI_FUNC(PyObject *) PyObject_VectorcallMethod(
 static inline PyObject *
 PyObject_CallMethodNoArgs(PyObject *self, PyObject *name)
 {
-    return _PyObject_VectorcallMethod(name, &self,
+    return PyObject_VectorcallMethod(name, &self,
            1 | PY_VECTORCALL_ARGUMENTS_OFFSET, NULL);
 }
 
@@ -187,7 +187,7 @@ PyObject_CallMethodOneArg(PyObject *self, PyObject *name, PyObject *arg)
 {
     assert(arg != NULL);
     PyObject *args[2] = {self, arg};
-    return _PyObject_VectorcallMethod(name, args,
+    return PyObject_VectorcallMethod(name, args,
            2 | PY_VECTORCALL_ARGUMENTS_OFFSET, NULL);
 }