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] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
         python-version: ['3.9', '3.10', '3.11', '3.12']
     runs-on: ${{ matrix.platform }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v6
       with:
@@ -40,7 +40,7 @@ jobs:
         tox-env: [build, docs, lint]
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Set up Python 3.12
       uses: actions/setup-python@v6
       with:

.github/workflows/docs.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Set up Python 3.12
         uses: actions/setup-python@v6
         with:

.github/workflows/release.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: 'ubuntu-latest'
     steps:
     - name: Checkout
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
     - name: Set up Python
       uses: actions/setup-python@v6
       with: