reduce CI matrix to mainstream Python versions

- Remove Python 3.9 support to focus on mainstream versions
- Keep Python 3.10, 3.11, and 3.12 for better coverage/performance balance

Author: BohuTANG

.github/workflows/python.yml

@@ -22,7 +22,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.9", "3.10", "3.11", "3.12"]
+        python-version: ["3.10", "3.11", "3.12"]
     steps:
       - uses: actions/checkout@v4
       - name: Setup Python ${{ matrix.python-version }}