Add pillow install to GitHub Actions.

Author: python-processing-unit

.github/workflows/run-tests.yml

@@ -20,10 +20,10 @@ jobs:
         with:
           python-version: "3.14"
 
-      - name: Install numpy and mypy
+      - name: Install numpy, pillow, and mypy
         run: |
           python -m pip install --upgrade pip
-          pip install numpy mypy
+          pip install numpy pillow mypy
         shell: pwsh
 
       - name: Run test with asm-lang.exe (if present)