Add METH_FASTCALL calling convention

Issue #27810: Add a new calling convention for C functions:

    PyObject* func(PyObject *self, PyObject **args,
                   Py_ssize_t nargs, PyObject *kwnames);

Where args is a C array of positional arguments followed by values of keyword
arguments. nargs is the number of positional arguments, kwnames are keys of
keyword arguments. kwnames can be NULL.

Python/getargs.c

@@ -1992,8 +1992,9 @@ vgetargskeywordsfast(PyObject *args, PyObject *keywords,
                 return cleanreturn(0, &freelist);
             }
         }
-        else if (i < nargs)
+        else if (i < nargs) {
             current_arg = PyTuple_GET_ITEM(args, i);
+        }
 
         if (current_arg) {
             msg = convertitem(current_arg, &format, p_va, flags,