Code review comment for ubuntu-debuginfod:poller

I have several suggestions for improvements but none that I feel to be critical enough to block on so set as approved, and you can take or leave the suggestions as desired. The code looks good, this will add a nice functionality!

Don't forget to also include the python3-launchpad dependency in the README.md. (Might not hurt to have some discussion in README.md or somewhere about how these new classes are used, but maybe that comes later.)