Update book.yml

change python version to 3.9.21

Author: sukunis

.github/workflows/book.yml

@@ -15,10 +15,10 @@ jobs:
     - uses: actions/checkout@v2
 
     # Install dependencies
-    - name: Set up Python 3.7
+    - name: Set up Python 
       uses: actions/setup-python@v1
       with:
-        python-version: 3.7
+        python-version: 3.9.21
 
     - name: Install dependencies
       run: |