Struct class lives in _struct (#12980)

This one is an improvement on 3.9+. On 3.8, the Struct class
calls itself `builtins.Struct` instead, which we can't and won't
match. `struct.error` is defined in `_struct.c`, but always called
itself `struct.error`.

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

stdlib/VERSIONS

@@ -61,6 +61,7 @@ _socket: 3.0-  # present in 3.0 at runtime, but not in typeshed
 _sqlite3: 3.0-
 _ssl: 3.0-
 _stat: 3.4-
+_struct: 3.0-
 _thread: 3.0-
 _threading_local: 3.0-
 _tkinter: 3.0-