CI: install Pillow for building docs

Author: swt2c

.github/workflows/ci-build.yml

@@ -237,6 +237,7 @@ jobs:
       - name: Install Python dependencies
         run: |
           python -m pip install --upgrade -r requirements.txt
+          python -m pip install pillow
 
       - name: Setup MSVC
         uses: ilammy/msvc-dev-cmd@v1