Abhilash Raj (maxking) has made a change to the installation documentation for setting up Mailman 3 in a virtual env (switching from uwsgi to http) and that gets my system further.
I'm now at the point where Mark's suggestion regarding urlpatterns is the blocker, in that querying "https://example.com/archives/api/mailman/urls" works but "https://example.com/hyperkitty/api/mailman/urls" doesn't.
As I noted in my previous reply, I don't have a file /opt/mailman/mm/urls.py on my system. I have found venv/lib/python3.8/site-packages/mailman_web/urls.py that looks to be the file that needs to be updated. I'm on the latest version of that package, 0.0.4.
If I hack that urls.py file to include the hyperkitty URL and send a new message, it all works!
Regards
Philip