bpo-39493: Fix definition of IO.closed in typing.py (GH-18265)

(cherry picked from commit 2e6569b6692298fcc9aae0df3eb3181adb2a5099)

Co-authored-by: Shantanu <hauntsaninja@users.noreply.github.com>
diff --git a/Lib/typing.py b/Lib/typing.py
index 69f6d98..83d310f 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1848,6 +1848,7 @@
     def close(self) -> None:
         pass
 
+    @property
     @abstractmethod
     def closed(self) -> bool:
         pass