Attempt to fix docs pipeline

Author: wesselb

.github/workflows/build_book.yml

@@ -13,13 +13,13 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        python-version: [3.8]
+        python-version: [3.9]
     steps:
     - uses: actions/checkout@v2
 
     # Install dependencies.
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v1
+      uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies