Comment 9 for bug 343046

Reading a posting to the mailman list I found this:

http://<email address hidden>/msg51844.html

"Also, you may be able to work around the problem by just removing the
pythonlib/email/ directory from your Mailman installation. This won't
stop the deprecation warnings, but it may allow your current Mailman
to run."

I therefore deleted the directory and the removed the package:

sudo rm -rf /usr/lib/mailman/pythonlib/email
sudo apt-get remove mailman

The package was successfully removed. I guess I could have allowed the upgrade to go ahead but I was testing!

The tasks held back from the do-release-upgrade then continued automatically after the package had been removed.