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

Objects/longobject.c

@@ -5547,7 +5547,7 @@ int_from_bytes_impl(PyTypeObject *type, PyObject *bytes_obj,
     Py_DECREF(bytes);
 
     if (long_obj != NULL && type != &PyLong_Type) {
-        Py_SETREF(long_obj, _PyObject_CallOneArg((PyObject *)type, long_obj));
+        Py_SETREF(long_obj, PyObject_CallOneArg((PyObject *)type, long_obj));
     }
 
     return long_obj;