Improve typing for import_from_yaml and export_to_yaml

[Morwenn]

Hi, thanks for the great library.

I regularly need to send pathlike.Path objects to the filepath parameter of import_from_yaml. It does already work correctly because filepath is simply forwarded to open, but it breaks the typing, forcing me to either use #type: ignore or to convert the path to a string explicitly, which becomes a bit noisy when there's many occurrences.

This MR changes the expected type of filepath from str to Union[str, bytes, os.PathLike], mirroring what open accepts.

I modified export_to_yaml as a drive-by fix for consistency, since it also just forwards its parameter to open.

[AlexandreDecan]

Hello,

Thanks for the suggested change and the explanations. Are there other places where a filepath is expected and accepting a Path object makes sense? Could you check and apply your change, so that we uniformly and consistently accept Path objects as well?

Thanks!

[Morwenn]

I searched for all instances of open( in the library, and could only find export_to_plantuml that warranted a similar changed. Which I applied accordingly.

[AlexandreDecan]

Thanks, I'll also check on my side (I think there are a few other places where we support file paths such as in the bdd cli). I'll be away for a few days so don't worry if this pr remains open for now :-)