Use stable Python 3.15 in CI

.github/workflows/tests.yml

@@ -32,7 +32,6 @@ jobs:
         uses: actions/setup-python@v7
         with:
           python-version: ${{ matrix.python-version }}
-          allow-prereleases: ${{ matrix.python-version == '3.15' }}
       - name: Install Python dependencies
         run: |
           uv sync --locked --group dev