ci: bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/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
...

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

.github/workflows/python-unit-test.yml

@@ -40,7 +40,7 @@ jobs:
           sudo apt install -y git
 
       - name: checkout PR
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Set up Python 2.7
         if: ${{ matrix.pyver_os.ver == '2.7' }}
@@ -95,7 +95,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: checkout PR
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Run py26 tests
         uses: linux-system-roles/lsr-gh-action-py26@1.0.2