Better typing of `Element`

[lord-haffi]

This is a follow-up issue on #80 and #81. When reintroducing the py.typed file (#82), I get some type errors related to the following problem.

There are many fields typed using the Element = str | list[str] type. This creates some annoying problems as many fields like segment.tag or message.receiver are now str | list[str] while in realitiy it should always be str, shouldn't it?