gh-87613: Use Argument Clinic @vectorcall for decimal.Decimal

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: cmaloney

Modules/_decimal/_decimal.c

@@ -3314,6 +3314,7 @@ PyDec_FromObject(PyObject *v, PyObject *context)
 
 /*[clinic input]
 @classmethod
+@vectorcall
 _decimal.Decimal.__new__ as dec_new
 
     value: object(c_default="NULL") = "0"
@@ -3329,7 +3330,7 @@ trap is active.
 
 static PyObject *
 dec_new_impl(PyTypeObject *type, PyObject *value, PyObject *context)
-/*[clinic end generated code: output=35f48a40c65625ba input=5f8a0892d3fcef80]*/
+/*[clinic end generated code: output=35f48a40c65625ba input=dd1efa0c0ffe9416]*/
 {
     decimal_state *state = get_module_state_by_def(type);
     CONTEXT_CHECK_VA(state, context);
@@ -6190,6 +6191,7 @@ static PyType_Slot dec_slots[] = {
     {Py_tp_methods, dec_methods},
     {Py_tp_getset, dec_getsets},
     {Py_tp_new, dec_new},
+    {Py_tp_vectorcall, dec_vectorcall},
 
     // Number protocol
     {Py_nb_add, nm_mpd_qadd},