ci: bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...v6)

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

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

Author: dependabot[bot]

.github/workflows/publish-to-pypi.yml

@@ -8,7 +8,7 @@ jobs:
     build:
         runs-on: ubuntu-latest
         steps:
-            - uses: actions/checkout@v4
+            - uses: actions/checkout@v6
             - name: Set up Python
               uses: actions/setup-python@v5
               with:

.github/workflows/test.yml

@@ -14,7 +14,7 @@ jobs:
             matrix:
                 python-version: ["3.12", "3.13"]
         steps:
-            - uses: actions/checkout@v4
+            - uses: actions/checkout@v6
             - name: Set up Python ${{ matrix.python-version }}
               uses: actions/setup-python@v5
               with: