add optional extra to testing

Author: tschaume

.github/workflows/testing.yml

@@ -50,7 +50,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install -e .[tests]
+          pip install -e .[tests,optional]
 
       - name: Test Modules
         run: pytest --cov=pymatgen --cov-report=xml