Prepare release: 3.1.0

[github-actions[bot]]

⚡ Merging this PR will automatically create a GitHub Release & publish the package ⚡

This PR was created by the Release stage 1 - create release PR workflow, triggered by @MetRonnie

Tests:

• Tests last run on master:
• ✔️ Build check passed - see the workflow run (number 3) for more info

Checklist:

• [x] Milestone complete? 1 other open issues/PRs on milestone 3.1.0 at time of PR creation
• [x] Changelog up-to-date?
  • Examine pull requests made since the last release
  • "Released on" date updated? ✔️ 2023-10-05

Next steps:

• @MetRonnie should request 1 or 2 reviews
• If any further changes are needed, push to this PR branch
• After merging, the bot will comment below with a link to the release (if not, look at the PR checks tab)

[MetRonnie]

Other open issue on this milestone is https://github.com/metomi/isodatetime/issues/195

[oliver-sanders]

(bumped the other issue)

[github-actions[bot]]

✔️ success

Next steps for @MetRonnie:

• view/edit the GitHub release description as appropriate
• ✔️ Milestone automatically closed

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Comment created by workflow: Release stage 2 - auto publish, run: 3