fix: add missing __future__ import to builtin_collectors.py for Python 3.9

Author: tom-sapletta-com

pyqual/builtin_collectors.py

@@ -1,5 +1,7 @@
 """Built-in metric collector plugins for pyqual."""
 
+from __future__ import annotations
+
 import json
 from pathlib import Path
 from typing import Any