CI: Set up CI for the free-threaded build of 3.13
Related to #59057.
Good news is free-threading CI is passing! 🎉 

Bad news is there's failures in `Docstring validation, typing, and other manual pre-commit hooks` and `windows-latest actions-312.yaml`, but I don't know how these can be related to this PR (they also appear to be passing on latest `main`). Both are unrelated to this PR. Windows was flaky and got green after a rerun. Opened a new issue about the doctest failures in #59093. Thanks @lysnikolaou 