Comment 7 for bug 2067196

> Summarizing, the key is python3-urllib3: we're syncing it from experimental

Aha, that makes sense. Out of curiosity, how did you track down this specific dependency? Because doing it manually would require to check all build dependencies and in turn their dependencies to find the difference between Ubuntu and Debian.

Looks like in the meantime the new version of python3-urllib3 has been uploaded to unstable, which caused the same build failure there (see attached bug watch). They solved it in a different way which was patching out the useage of python3-six all togheter. I've therefore filed bug 2070056 to get the package in sync again.

Thanks for review, and especially the root cause explanation :)