Add cffi to CI

.github/workflows/ci.yml

@@ -30,6 +30,7 @@ jobs:
         python -m pip install --upgrade pip
         pip install --upgrade setuptools
         pip install pytest pre-commit
+        pip install cffi
     - name: Install dependencies for Python
       run: |
         pip install -c requirements.txt -e .[tests,sphinx,svgsupport,aafiguresupport,mathsupport,rawhtmlsupport]