Typing fix. (#3759)

Author: rodrigogiraoserrao

src/textual/_box_drawing.py

@@ -12,11 +12,13 @@
 
 """
 
+from __future__ import annotations
+
 from functools import lru_cache
 
 from typing_extensions import TypeAlias
 
-Quad: TypeAlias = tuple[int, int, int, int]
+Quad: TypeAlias = "tuple[int, int, int, int]"
 """Four values indicating the composition of the box character."""
 
 # Yes, I typed this out by hand. - WM