[CI] Align Ruff target with Python 3.9 floor

The package now requires Python 3.9+, so Ruff should target py39 for pyupgrade and lint behavior.

Author: tqchen

pyproject.toml

@@ -181,7 +181,7 @@ testpaths = ["tests"]
 include = ["python/**/*.py", "tests/**/*.py"]
 line-length = 100
 indent-width = 4
-target-version = "py38"
+target-version = "py39"
 
 [tool.ruff.lint]
 select = [