Allow prereleases in CI for Python 3.15 beta

Author: neuhaus

.github/workflows/ci.yml

@@ -21,6 +21,7 @@ jobs:
       uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python-version }}
+        allow-prereleases: true
     - uses: actions/cache@v4
       with:
         path: ~/.cache/pip