Update testing.yml

Author: tschaume

.github/workflows/testing.yml

@@ -78,7 +78,7 @@ jobs:
 
       - name: Install dependencies
         run: |
-          python -m pip install --upgrade pip
+          python -m pip install --upgrade pip lxml[html_clean]
           pip install -e .[docs]
       - name: Build
         run: sphinx-build docs/source _build