fix: skip staticmethod for python3.9

Author: Quek Ching Yee

tests/tree/test_export.py

@@ -1140,11 +1140,11 @@ def test_tree_to_polars_name_col_missing(tree_node):
         actual = export.tree_to_polars(tree_node, name_col="")
         assert expected.equals(actual)
 
+    @staticmethod
     @unittest.skipIf(
         tuple(map(int, pl.__version__.split(".")[:2])) > (1, 9),
         reason="Not compatible with polars>1.9.0",
     )
-    @staticmethod
     def test_tree_to_polars_name_path_col_missing(tree_node):
         expected = pl.DataFrame()
         expected.index = range(8)