Add a comment for checking UCS-4 in for Python

Co-Authored-By: qiuzhong <[email protected]>

css/build-css-testsuites.sh

@@ -35,6 +35,7 @@ main() {
             exit 1
         fi
 
+        # The maximum Unicode code point is U+10FFFF = 1114111
         if [ `$PYTHON -c 'import sys; print(sys.maxunicode)'` != "1114111" ]; then
             echo "UCS-4 support for Python is required"
             exit 1