revert back to actions v4 to keep this branch clean

Author: sgd2z

.github/workflows/ci.yaml

@@ -22,7 +22,7 @@ jobs:
     env:
       PYTHON: ${{ matrix.python-version }}
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 0 # fetch all history for setuptools_scm to be able to read tags