I am also trying to install mailman3 on ubuntu 20.04 I followed https://gist.github.com/plepe/dab22fdbfec63d8632709065890124a3
I found that apache2 proxy_uwsgi package is missing the proxy_uwsgi.load file although the .so file is already there.
Also the mailman3-full that comes with 20.04 has the python <3.8 bug with time.clock().
I decided to just use pip virtualenv installation method instead - https://docs.mailman3.org/en/stable/prodsetup.html. It's working fine so far.
Now I am trying to figure out how to configure mailman+postfix to pass the unnecessarily convoluted world of spam-free certification and ip blockings by those 'security lawmakers'.
Any suggestions on how to not be blocked by gmail, hotmail, etc? I have used DKIM (no signing yet), SPF, DMARC to no avail. Maybe not an appropriate question to this mailman list.