This is by design; see PEP 475, and the documentation <https://docs.python.org/3.5/library/time.html#time.sleep>.

If you make your signal handler raise an exception, it will interrupt the sleep() call most of the time. But if the signal happens to be received just before the sleep() call is about to be entered, the handler will only be run when the underlying OS sleep() call returns 10 s later.