The API of update-manager changed so it's not possible anymore to import the MetaReleaseCore object since it doesn't exist.
The documentation of the new API is available here:
This affects both APT backends so it would be great if you could update this code so it can use the new API update-manager exposes.
Thanks and best regards,
We moved the upstream bugtracker to GitHub a long time ago. If this issue still affects you please re-create the issue here: https://github.com/hughsie/PackageKit/issues
Sorry for the impersonal message, and fingers crossed your issue no longer happens. Thanks.