[WIP] ci: Python 3.13 support

[toby-coleman]

Summary

Closes #29 with lint/test CI stages running on both Python 3.12 and Python 3.13. Build stage requires no change as of now, because none of our code is compiled against a specific version. If this changes in future then it is straightforward to update to matrix builds on the PyPI wheel CI job.

Changes

• Add matrix build for Python 3.12 and Python 3.13. See output here.
• Update Github codespace to use precommit hook.

[toby-coleman]

TODO:

• Currently the tests fail because msgspec has not been updated for Python 3.13 - there are active Github issues on this: Build Python 3.13 wheels (not free-threaded) jcrist/msgspec#711. If this becomes a blocker we can swap out msgspec for its underlying YAML parser if needed.
• Revert branch condition on CI pipeline.

[toby-coleman]

Abandoning in favour of #59.