Merge pull request #265 from DoubleML/s-small-bugfixes

Update python version in workflow

Author: SvenKlaassen

.github/workflows/pytest.yml

@@ -56,7 +56,7 @@ jobs:
     - name: Test with pytest
       if: |
         matrix.config.os != 'ubuntu-latest' ||
-        matrix.config.python-version != '3.8'
+        matrix.config.python-version != '3.9'
       run: |
         pytest -m ci