Upgrade github workflow to node16

This solves the warning message in CI: "The following actions uses node12 which is deprecated and will be forced to run on node16: actions/checkout@v2. For more info: https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/".

.github/workflows/format_code.yaml

@@ -4,7 +4,7 @@ jobs:
   format-code:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: psf/black@stable
         with:
           options: "--check --diff"