Update book.yml

Author: haesleinhuepf

.github/workflows/book.yml

@@ -15,10 +15,10 @@ jobs:
     - uses: actions/checkout@v2
 
     # Install dependencies
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v1
       with:
-        python-version: 3.8
+        python-version: 3.9
 
     - name: Install dependencies
       run: |