add write permissions to build-and-deploy job

otherwise sphinx-build fails with permission error, probably due to recent GitHub changes

.github/workflows/sphinx.yml

@@ -12,6 +12,8 @@ jobs:
   build-and-deploy:
     name: Build and gh-pages
     runs-on: ubuntu-latest
+    permissions:
+      contents: write    
     steps:
       # https://github.com/marketplace/actions/checkout
       - uses: actions/checkout@v3