chore(deps): update actions/setup-python action to v6 (#504)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [actions/setup-python](https://github.com/actions/setup-python) | action | major | `v5` -> `v6` |

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### Release Notes

<details>
<summary>actions/setup-python (actions/setup-python)</summary>

### [`v6`](https://github.com/actions/setup-python/compare/v5...v6)

[Compare Source](https://github.com/actions/setup-python/compare/v5...v6)

</details>

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### Configuration

📅 **Schedule**: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻ **Rebasing**: Whenever PR is behind base branch, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

---

 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box

---

This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate).
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Reviewed-on: #504
Co-authored-by: Renovate [BOT] <renovate-bot@git.tainton.uk>
Co-committed-by: Renovate [BOT] <renovate-bot@git.tainton.uk>

.gitea/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
           no-fail: true
 
       - name: Setup Python
-        uses: actions/setup-python@v5
+        uses: actions/setup-python@v6
         with:
           python-version: "3.13"