Add codecs.BOM* constants (#6515)

stdlib/codecs.pyi

@@ -5,6 +5,11 @@ from abc import abstractmethod
 from typing import IO, Any, BinaryIO, Callable, Generator, Iterable, Iterator, Protocol, TextIO, Tuple, Type, TypeVar, overload
 from typing_extensions import Literal
 
+BOM32_BE: bytes
+BOM32_LE: bytes
+BOM64_BE: bytes
+BOM64_LE: bytes
+
 # TODO: this only satisfies the most common interface, where
 # bytes is the raw form and str is the cooked form.
 # In the long run, both should become template parameters maybe?