Run Argument Clinic: METH_VARARGS=>METH_FASTCALL

Issue #29286. Run Argument Clinic to get the new faster METH_FASTCALL calling
convention for functions using "boring" positional arguments.

Manually fix _elementtree: _elementtree_XMLParser_doctype() must remain
consistent with the clinic code.

Modules/_elementtree.c

@@ -2743,7 +2743,7 @@ typedef struct {
 } XMLParserObject;
 
 static PyObject*
-_elementtree_XMLParser_doctype(XMLParserObject* self, PyObject* args);
+_elementtree_XMLParser_doctype(XMLParserObject *self, PyObject **args, Py_ssize_t nargs, PyObject *kwnames);
 static PyObject *
 _elementtree_XMLParser_doctype_impl(XMLParserObject *self, PyObject *name,
                                     PyObject *pubid, PyObject *system);