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?

[lord-haffi]

Also, mypy has problems to infer the type of e.g. Segment.elements correctly. As it is defined through elements = None on class-level, mypy will think that this field can actually be None. Which is obviously wrong when you look at the logic inside __init__.

[nerdoc]

This may be also due to the fact that the whole process of parsing is upgraded using "typed" classes with a factory pattern, like segments. So everything is a bit "in progress" now.

See #92

[tammoippen]

fields like segment.tag or message.receiver are now str | list[str] while in realitiy it should always be str, shouldn't it?

I think you mean Interchange.recipient? And sender and recipient can actually be list[str] - we have a use case for this.

segment.tag should be a str with the current version.

[nerdoc]

yes, Segment.tag must be always a str. But Interchange.recipient/sender is actually always a list[str] technically, because:

020 	S002 	INTERCHANGE SENDER 	M 	1 		
  0004 	Interchange sender identification 	M 		an..35 	
  [0007](https://www.stylusstudio.com/edifact/40100/0007.htm) 	Identification code qualifier 	C 		an..4 	
  0008 	Interchange sender internal identification 	C 		an..35 	
  0042 	Interchange sender internal sub-identification 	C 		an..35 	
030 	S003 	INTERCHANGE RECIPIENT 	M 	1 		
  0010 	Interchange recipient identification 	M 		an..35 	
  [0007](https://www.stylusstudio.com/edifact/40100/0007.htm) 	Identification code qualifier 	C 		an..4 	
  0014 	Interchange recipient internal identification 	C 		an..35 	
  0046 	Interchange recipient internal sub-identification 	C 		an..35

So in the final version, "Sender" will be a Composite Element named InterchangeSender that has a .InterchangeSenderIdentification property etc.

I'm currently writing a parser for the UN homepages to extract all segments and elements and produce boilerplate code from it. Works great. I'm just lacking the ability of being at two or more places at the same time, so progress is slow. :-)