Fix missing field initializer in (Python 3.12)

In Python 3.12, a new internal-only field was added to PyTypeObject,
which must be initialized in ClassDefinition. Both GCC and Clang
complained with:

  warning: missing initializer for member '_typeobject::tp_watched' [-Wmissing-field-initializers]

lass/python/class_definition.cpp

@@ -161,6 +161,9 @@ ClassDefinition::ClassDefinition(
 		0,	/*tp_vectorcall, exists in 3.8 but only used since 3.9 */
 #if PY_VERSION_HEX >= 0x03080000 && PY_VERSION_HEX < 0x03090000 // == 3.8
         0,	/*tp_print, only exists in 3.8 for backwards compatibility, bpo-37250 */
+#endif
+#if PY_VERSION_HEX >= 0x030C0000 // >= 3.12
+		0,	/*tp_watched, internal only */
 #endif
 	};
 	type_ = type;