Fix attribute error warning in type checking.

Author: ChenPi11

rubisco/lib/pathlib.py

@@ -23,6 +23,7 @@
 import os
 import pathlib
 import sys
+from typing import TYPE_CHECKING
 
 __all__ = ["Path"]
 
@@ -41,7 +42,7 @@ def normpath(self) -> "Path":
 
 
 # In Python 3.11, parent's @classmethod will catch children.
-if sys.version_info < (3, 12):
+if sys.version_info < (3, 12) and not TYPE_CHECKING:
     # "Inject" the extended methods into the built-in pathlib.Path class.
     pathlib.Path.normpath = Path.normpath  # type: ignore[method-required]
     Path = pathlib.Path  # type: ignore[assignment]