Fix coverage

Author: jan-janssen

.github/workflows/unittests.yml

@@ -71,6 +71,7 @@ jobs:
         pip install versioneer[toml]==0.29
         pip install . --no-deps --no-build-isolation
         coverage run  --omit="structuretoolkit/_version.py,tests/*" -m unittest discover tests
+        coverage xml
     - name: Coverage
       if:  matrix.label == 'linux-64-py-3-12'
       uses: coverallsapp/github-action@v2