actions: 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/pytest-not-slow.yml

@@ -19,7 +19,7 @@ jobs:
       configurations: ${{ steps.matrix.outputs.configurations }}
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Set up Python
         uses: actions/setup-python@v6
@@ -57,7 +57,7 @@ jobs:
     runs-on: ${{ matrix.configuration.os }}
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
 
     - name: Set up Python ${{ matrix.configuration.python-version }}
       uses: actions/setup-python@v6

.github/workflows/pytest-slow.yml

@@ -20,7 +20,7 @@ jobs:
         scipy-version: ['scipy~=1.14.0']
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
 
     - name: Set up Python ${{ matrix.os-python.python-version }}
       uses: actions/setup-python@v6