Ubuntu Release Management

For the removal of the old Python version, proceed as follows:
Upload the python3-defaults package, cancel all triggered builds,
force it into the release pocket, wait until it is published and visable
in the release pocket, then re-upload the python3-defaults package for
retriggering the autopkg tests. Doing all the no-change uploads before that
would still trigger autopkg tests for the no-change uploads with the old python
version.
Once python3-defaults has migrated, the binNMU ordering is no longer important,
except that boostN.MM must be done early to avoid FTBFS in packages trying to
link mismatching libpython and libboost-python.
Other packages that do not show up in the tracker are pycxx, ...