ci: add Pillow to test dependencies (required by thumbnails module)

Author: GeiserX

.github/workflows/tests.yml

@@ -23,7 +23,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install -r requirements.txt
-          pip install pytest pytest-cov beautifulsoup4
+          pip install pytest pytest-cov beautifulsoup4 Pillow
       
       - name: Run tests
         run: |