Use Python 3.12 for the 3.10 docs build

The pinned version of Sphinx requires `imghdr`, removed in Python 3.13.

Author: encukou

.github/workflows/doc.yml

@@ -38,7 +38,7 @@ jobs:
     - name: 'Set up Python'
       uses: actions/setup-python@v4
       with:
-        python-version: '3'
+        python-version: '3.12'
         cache: 'pip'
         cache-dependency-path: 'Doc/requirements.txt'
     - name: 'Install build dependencies'