fix: align Python version requirements to 3.9

- Update root project to support Python 3.9, matching subpackages
- Restore macOS Python 3.9 support in CI
- This fixes the CI failure for Python 3.9 environments

pyproject.toml

@@ -5,7 +5,7 @@ build-backend = "setuptools.build_meta"
 [project]
 name = "leann-workspace"
 version = "0.1.0"
-requires-python = ">=3.10"
+requires-python = ">=3.9"
 
 dependencies = [
     "leann-core",