Use _typeshed.Self with __enter__ (#5719)

stdlib/winreg.pyi

@@ -1,3 +1,4 @@
+from _typeshed import Self
 from types import TracebackType
 from typing import Any, Optional, Tuple, Type, Union
 
@@ -90,7 +91,7 @@ error = OSError
 class HKEYType:
     def __bool__(self) -> bool: ...
     def __int__(self) -> int: ...
-    def __enter__(self) -> HKEYType: ...
+    def __enter__(self: Self) -> Self: ...
     def __exit__(
         self, exc_type: Optional[Type[BaseException]], exc_val: Optional[BaseException], exc_tb: Optional[TracebackType]
     ) -> Optional[bool]: ...