Use PEP 604 syntax wherever possible (#7493)

stdlib/mailbox.pyi

@@ -3,7 +3,7 @@ import sys
 from _typeshed import Self, StrOrBytesPath
 from abc import ABCMeta, abstractmethod
 from types import TracebackType
-from typing import IO, Any, AnyStr, Callable, Generic, Iterable, Iterator, Mapping, Protocol, Sequence, TypeVar, Union, overload
+from typing import IO, Any, AnyStr, Callable, Generic, Iterable, Iterator, Mapping, Protocol, Sequence, TypeVar, overload
 from typing_extensions import Literal
 
 if sys.version_info >= (3, 9):
@@ -31,7 +31,7 @@ __all__ = [
 
 _T = TypeVar("_T")
 _MessageT = TypeVar("_MessageT", bound=Message)
-_MessageData = Union[email.message.Message, bytes, str, IO[str], IO[bytes]]
+_MessageData = email.message.Message | bytes | str | IO[str] | IO[bytes]
 
 class _HasIteritems(Protocol):
     def iteritems(self) -> Iterator[tuple[str, _MessageData]]: ...