Comment 17 for bug 2007241

Lucas is right - the problem we have here is that the old yaml modules are importing `Hashable` from `collections` instead of `collections.abc`. The thing is - this was deprecated, but still retrocompatible until Python3.8 (which we do support in Focal)

In Python 3.9 it was officially dropped and starts breaking the old modules - and we only support 3.9 (3.10 actually) from Jammy onwards.

(See bpo-25988 in https://docs.python.org/3/whatsnew/changelog.html#id280