Fix Python 3.9-incompatible union syntax

Author: clayote

src/attrs/sphinx_cached_property.py

@@ -6,8 +6,10 @@
 classes will be inaccessible.
 
 """
+from __future__ import annotations
+
 from functools import cached_property
-from typing import Any
+from typing import Any, Union
 
 from sphinx.application import Sphinx