Mark Sapiro writes:
I only discovered this because I saw the ERROR entries in the log while looking for something else. In any case, it turned out that python-openssl was installed on mail.python.org and not on lists.mailman3.org. Removing it from mail.python.org allowed OAuth2 logins to work.
I think that part of the problem here may be that distros have not caught up to current Python upstream distribution practices. MacPorts is particularly bad (ie, heinous), in that they don't even follow their own Python distributions. In MacPorts, OpenSSL and GNU readline have both bitten me; basically, the add-on packages should be labeled "unless you know you need this, and know why, and accept that NO WARRANTEE means what it says (we don't support it at all), don't install this!"
I've opened Mailman/Mailman issue#323 for this. Probably it should be moved to Installation and Troubleshooting documentation, but maybe leaving an open issue in the tracker would help discoverability?