Bump actions/checkout in /.github/workflows in the actions group

Bumps the actions group in /.github/workflows with 1 update: [actions/checkout](https://github.com/actions/checkout).


Updates `actions/checkout` from 5 to 6
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: actions
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/publish.yml

@@ -16,7 +16,7 @@ jobs:
     if: ((github.event_name == 'push' && startsWith(github.ref, 'refs/tags')) || contains(github.event.pull_request.labels.*.name, 'Build wheels'))
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0
     - uses: actions/setup-python@v6

.github/workflows/python-tests.yml

@@ -19,7 +19,7 @@ jobs:
         python-version: ["3.11", "3.12", "3.13", "3.14"]
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0
     - name: Set up Python ${{ matrix.python-version }}