ci: target Python 3.9 for testing

Update CI to use Python 3.9 as the test version. This ensures
compatibility with Python 3.9+ as specified in pyproject.toml
(requires-python = ">=3.8").

Python 3.9 is a good balance between:
- Modern features and performance
- Wide compatibility (still supported until Oct 2025)
- Common in enterprise environments

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: aepfli

.github/workflows/ci.yml

@@ -121,7 +121,7 @@ jobs:
 
       - uses: actions/setup-python@v5
         with:
-          python-version: '3.10'
+          python-version: '3.9'
 
       - name: Install uv
         uses: astral-sh/setup-uv@v4