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

(cherry picked from commit 2e6569b)

Co-authored-by: Shantanu <[email protected]>

Author: miss-islington

Lib/typing.py

@@ -1848,6 +1848,7 @@ def name(self) -> str:
     def close(self) -> None:
         pass
 
+    @property
     @abstractmethod
     def closed(self) -> bool:
         pass

Misc/NEWS.d/next/Library/2020-01-30-01-13-19.bpo-39493.CbFRi7.rst

@@ -0,0 +1 @@
+Mark ``typing.IO.closed`` as a property