Don't cancel other jobs from the 3.12 job failing

Because 3.12 is still a release candidate and if tests fail for it
then one would always want to know if/how other versions also fail.

This also allows actions/setup-python to install a prerelease for
3.12 only, not for other releases.

.github/workflows/pythonpackage.yml

@@ -16,6 +16,11 @@ jobs:
     strategy:
       matrix:
         python-version: ["3.8", "3.9", "3.10", "3.11", "3.12"]
+        include:
+          - experimental: false
+          - python-version: "3.12"
+            experimental: true
+    continue-on-error: ${{ matrix.experimental }}
 
     steps:
     - uses: actions/checkout@v4
@@ -25,7 +30,7 @@ jobs:
       uses: actions/setup-python@v4
       with:
         python-version: ${{ matrix.python-version }}
-        allow-prereleases: true
+        allow-prereleases: ${{ matrix.experimental }}
     - name: Install project and dependencies
       run: |
         python -m pip install --upgrade pip