Fix docs build

[stes]

WIP.

Root issue is that typing.Union is no longer in the python intersphinx mapping (https://docs.python.org/3/objects.inv), most likely due to this change (https://docs.python.org/3/library/typing.html#typing.Union):

We have to make a general decision which version of python to build against for the docs. I see two options:

1. The latest python version, this is right now 3.14: https://devguide.python.org/versions/.
2. A stable python version used in a linux distro we expect most users to have installed by default. As a first proxy, I guess Ubuntu LTS could be a suitable target, where python 3.12 is used: https://documentation.ubuntu.com/ubuntu-for-developers/reference/availability/python/

Whatever we do, I suggest we make a decision which version the docs are supposed to track. This PR right now goes with option (2), and updates everything to 3.12.

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Close #278
Fix #276

[stes]

All right, works: