[3.13] gh-126662: naming consistency for signal.ItimerError (GH-126712) (#137123)

gh-126662: naming consistency for `signal.ItimerError` (GH-126712)
(cherry picked from commit d5fa437)

Co-authored-by: Stephen Morton <[email protected]>

Author: miss-islington

Modules/signalmodule.c

@@ -1621,7 +1621,7 @@ signal_module_exec(PyObject *m)
     modstate->ignore_handler = state->ignore_handler;  // borrowed ref
 
 #ifdef PYHAVE_ITIMER_ERROR
-    modstate->itimer_error = PyErr_NewException("signal.itimer_error",
+    modstate->itimer_error = PyErr_NewException("signal.ItimerError",
                                                 PyExc_OSError, NULL);
     if (modstate->itimer_error == NULL) {
         return -1;