ci: add top-level permissions to test workflow

Author: deeleeramone

.github/workflows/test-pywry.yml

@@ -6,6 +6,9 @@ on:
     branches: [main, develop]
   workflow_dispatch:
 
+permissions:
+  contents: read
+
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true