stdlib: add __slots__ (#14611)

stdlib/ctypes/_endian.pyi

@@ -3,10 +3,14 @@ from ctypes import Structure, Union
 
 # At runtime, the native endianness is an alias for Structure,
 # while the other is a subclass with a metaclass added in.
-class BigEndianStructure(Structure): ...
+class BigEndianStructure(Structure):
+    __slots__ = ()
+
 class LittleEndianStructure(Structure): ...
 
 # Same thing for these: one is an alias of Union at runtime
 if sys.version_info >= (3, 11):
-    class BigEndianUnion(Union): ...
+    class BigEndianUnion(Union):
+        __slots__ = ()
+
     class LittleEndianUnion(Union): ...