add Pillow as test dependency

.github/workflows/testing.yml

@@ -24,7 +24,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         # Excluding Cython to test the pure Python version.
-        python -m pip install setuptools wheel pytest pyparsing fonttools numpy
+        python -m pip install setuptools wheel pytest pyparsing fonttools numpy Pillow
     - name: Install without C-extensions
       run: |
         python setup.py install