⬆️ 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/tests.yml

@@ -22,7 +22,7 @@ jobs:
         sphinx: ["", "~=7.0", "~=6.0", "~=5.0"]
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Set up Python
       uses: actions/setup-python@v5
       with:
@@ -46,7 +46,7 @@ jobs:
         sphinx: ["", "~=7.0", "~=5.0"]
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Set up Python
       uses: actions/setup-python@v5
       with:
@@ -86,7 +86,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Checkout source
-      uses: actions/checkout@v4
+      uses: actions/checkout@v6
     - name: Set up Python 3.8
       uses: actions/setup-python@v5
       with: