Add Python 3.15-dev to test matrix

Author: simonw

.github/workflows/test.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.10", "3.11", "3.12", "3.13", "3.14"]
+        python-version: ["3.10", "3.11", "3.12", "3.13", "3.14", "3.15-dev"]
     steps:
     - uses: actions/checkout@v6
     - name: Set up Python ${{ matrix.python-version }}