Just to complete the circle, this probably happens because the UpdateManager module is now only installed in the Python 3 sys.path, so when invoked with Python 2, the module isn't found.
Does it make sense to crash with a more useful message? E.g. we could put a sys.version test at the top of update-manager and complain loudly if it wasn't invoked with Python 3. OTOH, maybe this is good enough.
Yes, the bug is not in Update Manager, but in whatever is invoking u-m with the Python 2 command line.
Just to complete the circle, this probably happens because the UpdateManager module is now only installed in the Python 3 sys.path, so when invoked with Python 2, the module isn't found.
Does it make sense to crash with a more useful message? E.g. we could put a sys.version test at the top of update-manager and complain loudly if it wasn't invoked with Python 3. OTOH, maybe this is good enough.
Yes, the bug is not in Update Manager, but in whatever is invoking u-m with the Python 2 command line.