Merge pull request #311 from PyPSA/codecov-3

CI: use codecov-action V3

Author: fneum

.github/workflows/CI.yaml

@@ -35,4 +35,4 @@ jobs:
         pytest --cov=./  --cov-report=xml
     - name: Upload code coverage report
       if: matrix.os == 'ubuntu-latest'
-      uses: codecov/codecov-action@v1
+      uses: codecov/codecov-action@v3