The test failed at:

    # Fill the send buffer
    try:
        while True:
            write.send(b"x")
    except BlockingIOError:
        pass

    # By default, we get a warning when a signal arrives
    signal.set_wakeup_fd(write.fileno())

    with captured_stderr() as err:
        _testcapi.raise_signal(signum)

    err = err.getvalue()
    if ('Exception ignored when trying to {action} to the signal wakeup fd'
        not in err):
        raise AssertionError(err) # <~~~~~~ HERE