contextlib.nullcontext is a class (#5695)

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

Author: graingert

stdlib/contextlib.pyi

@@ -113,7 +113,11 @@ if sys.version_info >= (3, 7):
         ) -> Awaitable[bool]: ...
 
 if sys.version_info >= (3, 7):
-    @overload
-    def nullcontext(enter_result: _T) -> ContextManager[_T]: ...
-    @overload
-    def nullcontext() -> ContextManager[None]: ...
+    class nullcontext(AbstractContextManager[_T]):
+        enter_result: _T
+        @overload
+        def __init__(self: nullcontext[None], enter_result: None = ...) -> None: ...
+        @overload
+        def __init__(self: nullcontext[_T], enter_result: _T) -> None: ...
+        def __enter__(self) -> _T: ...
+        def __exit__(self, *exctype: Any) -> bool: ...