sed -i 's/from collections import/from collections.abc import/' /usr/lib/python3/dist-packages/mailmanclient/restbase/base.py
service mailman3-web restart
=> so this does seem to prove that the issue is that indeed the mailman3-web included with jammy is incomptible with previous mailman3. So supplying that elusive mailman3 package in jammy is indeed direly needed
Holding all packages containing mailman3 and also all packages containing django produces a (mostly) working mailman installation:
dpkg -l | awk '$2 ~ /mailman/ {print $2}' | xargs apt-mark hold
dpkg -l | awk '$2 ~ /django/ {print $2}' | xargs apt-mark hold
[ ... do upgrade ... ]
sed -i 's/from collections import/from collections.abc import/' /usr/lib/ python3/ dist-packages/ mailmanclient/ restbase/ base.py
service mailman3-web restart
=> so this does seem to prove that the issue is that indeed the mailman3-web included with jammy is incomptible with previous mailman3. So supplying that elusive mailman3 package in jammy is indeed direly needed