Run CI on ubuntu-24.04 so Python 3.9 and the pinned 3.10.4 resolve

The first run of the new pull_request matrix failed for Python 3.9
before any test ran: ubuntu-latest now resolves to the Ubuntu 26.04
image, and the actions/python-versions manifest has no 3.9 build for
it ("The version '3.9' with architecture 'x64' was not found for
Ubuntu 26.04"). 3.9 builds exist for 22.04 and 24.04 only. Python
3.10 to 3.13 passed on 26.04.

Pin the test matrix to ubuntu-24.04, where all five versions have
builds. The build-n-publish job pins Python 3.10.4, which also has no
26.04 build (only 20.04, 22.04 and 24.04), so it would fail at setup
on the next push to master or tag; pin its runner to ubuntu-24.04 too,
leaving its steps and the tag gate untouched.

Claude-Session: https://claude.ai/code/session_01VQiRp4zEJKUXHrKkFaeUke
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: claude

.github/workflows/python-publish.yml

@@ -11,7 +11,8 @@ on:
 jobs:
   test:
     name: Test (Python ${{ matrix.python-version }})
-    runs-on: ubuntu-latest
+    # ubuntu-latest now resolves to Ubuntu 26.04, for which actions/setup-python has no Python 3.9 builds
+    runs-on: ubuntu-24.04
     strategy:
       fail-fast: false
       matrix:
@@ -33,7 +34,8 @@ jobs:
   build-n-publish:
     name: Build and publish to PyPI
     if: github.event_name == 'push'
-    runs-on: ubuntu-latest
+    # the pinned Python 3.10.4 below has no build for Ubuntu 26.04 either
+    runs-on: ubuntu-24.04
     steps:
     - uses: actions/checkout@v4
     - name: Set up Python 3.10.4