bpo-37337: Add _PyObject_VectorcallMethod() (GH-14228)

diff --git a/Objects/descrobject.c b/Objects/descrobject.c
index 806c0af..a0eb505 100644
--- a/Objects/descrobject.c
+++ b/Objects/descrobject.c
@@ -851,15 +851,22 @@
 };
 
 static PyObject *
-mappingproxy_get(mappingproxyobject *pp, PyObject *args)
+mappingproxy_get(mappingproxyobject *pp, PyObject *const *args, Py_ssize_t nargs)
 {
-    PyObject *key, *def = Py_None;
-    _Py_IDENTIFIER(get);
+    /* newargs: mapping, key, default=None */
+    PyObject *newargs[3];
+    newargs[0] = pp->mapping;
+    newargs[2] = Py_None;
 
-    if (!PyArg_UnpackTuple(args, "get", 1, 2, &key, &def))
+    if (!_PyArg_UnpackStack(args, nargs, "get", 1, 2,
+                            &newargs[1], &newargs[2]))
+    {
         return NULL;
-    return _PyObject_CallMethodIdObjArgs(pp->mapping, &PyId_get,
-                                         key, def, NULL);
+    }
+    _Py_IDENTIFIER(get);
+    return _PyObject_VectorcallMethodId(&PyId_get, newargs,
+                                        3 | PY_VECTORCALL_ARGUMENTS_OFFSET,
+                                        NULL);
 }
 
 static PyObject *
@@ -894,7 +901,7 @@
             to the underlying mapping */
 
 static PyMethodDef mappingproxy_methods[] = {
-    {"get",       (PyCFunction)mappingproxy_get,        METH_VARARGS,
+    {"get",       (PyCFunction)mappingproxy_get,        METH_FASTCALL,
      PyDoc_STR("D.get(k[,d]) -> D[k] if k in D, else d."
                "  d defaults to None.")},
     {"keys",      (PyCFunction)mappingproxy_keys,       METH_NOARGS,