signalmodule.c uses _PyErr_WriteUnraisableMsg() (#98217)

Signal wakeup fd errors are now logged with
_PyErr_WriteUnraisableMsg(), rather than PySys_WriteStderr() and
PyErr_WriteUnraisable(), to pass the error message to
sys.unraisablehook. By default, it's still written into stderr (unless
sys.unraisablehook is overriden).