use https://github.com/grandcat/zeroconf

A Go library is a poor recommendation to substitute for a Python package.

All Python package that I found can't register a subtype

Simply add it to the service type string.

   info = ServiceInfo(
        "_somethingspecial._sub._http._tcp.local.",
        "Paul's Test Web Site._http._tcp.local.",
        addresses=[socket.inet_aton("127.0.0.1")],
        port=80,
        properties=desc,
        server="ash-2.local.",
    )

But my experience is that there is something no quite correct with the sub type handling.
Using avahi-browse -r _http._tcp to filter out the type wont show the above service when using python-zeroconf.
Using avahi-publish-service --subtype=_somethingspecial._sub._http._tcp basse _zmp._tcp 8080 will show just fine.