Update auto-publish workflow to Python 3.12 (#41)

See also:

Bikeshed Major Version updates (v6 and v7) and Python min version
update (3.12):
https://lists.w3.org/Archives/Public/spec-prod/2025OctDec/0004.html

Author: anssiko

.github/workflows/auto-publish.yml

@@ -11,6 +11,9 @@ jobs:
       contents: write
     steps:
       - uses: actions/checkout@v6
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
       - uses: w3c/spec-prod@v2
         with:
           GH_PAGES_BRANCH: gh-pages