fix: add future annotations for Python 3.9 compatibility

`dict | None` syntax requires Python 3.10+. Adding
`from __future__ import annotations` fixes collection on 3.9.

tests/test_cli_subprocess.py

@@ -9,6 +9,8 @@ catching issues that in-process CliRunner misses:
   - exit code propagation through the OS shell
 """
 
+from __future__ import annotations
+
 import json
 import os
 import subprocess