Enable Python 3.12

[Scienfitz]

(Written by Adrian) Enables Python 3.12 and fixes a bug with the validation converter, which also happens to fix the cattrs issue that originally blocked this PR (see details in PR comment).

[AdrianSosic]

Hi @Scienfitz, it took me one afternoon of debugging, but I eventually found the issue. Turned out it has nothing to do with cattrs (as far as I can tell) but is related to how Protocols are handled in Python 3.12, which causes and unintended fallback dispatch to our RecommenderProtocol. The issue is quite delicate and there are many unlucky things coming together (e.g., the validation then succeeds because for that class additional keywords are currently dropped, which is another open problem in our code for which we already have a longstanding TODO). Anyway, here the link to an issue I've posted that recreates the problem in form of a minimal example: python-attrs/cattrs#525

Regarding this PR: I made another minor change, which happens to fix the symptoms of the bug (for the root cause, we'll have to see what will happen in the ticket). The problem is fixed because the validation converter now contains an explicit hook for Campaign and therefore no longer relies on the incorrect fallback. However, note that the issue would return if we ever decided to remove that hook again, thus it's not a 100% satisfying solution. But for now, I think we can merge and track the ticket instead.

[AdrianSosic]

Can be merged but the required pipeline checks still need to be adjusted.

[AdrianSosic]

Solves #156