Update gh-actions workflows, use Python 3.12

.github/workflows/sphinx.yml

@@ -17,16 +17,16 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       # https://github.com/marketplace/actions/checkout
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 0
           lfs: true
       # https://github.com/marketplace/actions/setup-python
       # ^-- This gives info on matrix testing.
       - name: Install Python
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v4
         with:
-          python-version: 3.8
+          python-version: "3.12"
       # https://docs.github.com/en/actions/guides/building-and-testing-python#caching-dependencies
       # ^-- How to set up caching for pip on Ubuntu
       - name: Cache pip