Pin pillow everywhere, so png outputs stay the same

Author: bsipocz

.github/workflows/tests.yml

@@ -22,6 +22,7 @@ jobs:
         python-version: ["3.11", "3.12", "3.13", "3.14"]
         sphinx: [""]  # Newest Sphinx
         myst-parser: [""]  # Newest MyST Parser (any)
+        pillow: "==11.0.0"
         include:
           # Just check the other platforms once
           - os: windows-latest