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](actions/checkout@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>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -32,7 +32,7 @@ jobs:
     runs-on: ubuntu-latest
     container: docker://python:3.11-buster
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Build
         run: |
           python -m venv venv
@@ -274,7 +274,7 @@ jobs:
             reactor:
               tox: pyside2
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Enable Problem Matchers
         run: |
           echo "::add-matcher::.github/local-problem-matchers.json"
@@ -334,7 +334,7 @@ jobs:
           - short: 311
             dotted: "3.11"
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Set up Python ${{ matrix.python.dotted }}
         uses: actions/setup-python@v6
         with: