We use SIGUSR1/SIGUSR2 mainly in this test, but those signals are not
available on Windows:

        @func
        def test_signal():
            # Notify/Stop with wrong chan dtype -> panic
            _ = panics("pychan: channel type mismatch")
    >       with _:  signal.Notify(chan(2), syscall.SIGUSR1)
    E       AttributeError: module 'golang.syscall' has no attribute 'SIGUSR1'

-> Use SIGTERM/SIGINT if SIGUSR1/SIGUSR2 are not available.

Don't want to use SIGTERM/SIGINT unconditionally because those signals are
better to leave for the job control so that e.g. nxdtest can properly kill
spawned pygolang tests.