Update publish-book.yaml

.github/workflows/publish-book.yaml

@@ -58,6 +58,7 @@ jobs:
         run: |
           pip install -r requirements.txt
           pip install jupyter-book==0.14.0 ghp-import
+          pip install sphynx==5.0
           pip install jupyter_client==7.3.5  # downgrade jupyter-client to fix hangs
 
       - name: Build the book