Modernise to use type annotations rather than @rpc(Unicode, Integer, _returns=Iterable(Unicode))

[ghandic]

Modernizing could make spyne more user-friendly similar to FastAPI

https://fastapi.tiangolo.com/

[plq]

sure but it's a lot of work just for cosmetic reasons. we'll see.

[ghandic]

Seee #696 for an example without much effort