Enable Python 3.13 in CI and update action versions

[rffontenelle]

Updating action versions solves:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/setup-python@v4, codecov/codecov-action@v3. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

[lovelydinosaur]

This seems worth merging.
Blocked on unrelated issue #177.

[lovelydinosaur]

The build should be passing against this PR now, perhaps needs a nudge to re-run the rtd job.

[rffontenelle]

@tomchristie rebased, workflow just waiting approval to start and rtd passed