build(deps): bump actions/checkout from 6 to 6.0.3

Bumps [actions/checkout](https://github.com/actions/checkout) from 6 to 6.0.3.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v6...v6.0.3)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: 6.0.3
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

.github/workflows/main.yml

@@ -5,7 +5,7 @@ jobs:
     name: runner & black formatter
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v6.0.3
       - uses: psf/black@stable
         with:
           black_args: ". --check"