ci: use python version from pyproject.toml

Author: robtaylor

.github/workflows/build-docs.yml

@@ -16,7 +16,6 @@ jobs:
       - name: Set up PDM
         uses: pdm-project/setup-pdm@v4
         with:
-          python-version: "3.10"
           cache: true
 
       - name: Install deps