text: Title: Change paths passed to listener v3 methods to `pathlib.Path` instances
  Body: Listeners have methods like `output_file` and `log_file` that are called when result files are ready. At the moment they get the path to the file as a string, but I believe with listener v3 methods we should use [pathlib.Path](https://docs.python.org/3/library/pathlib.html) instead. `Path` instances are more convenient to use and we are now enhancing v3 listeners also otherwise.
  
  This change is backwards incompatible, but in most cases `str` and `Path` work the same way, so it's unlikely that the change causes big problems. For example, our acceptance tests passed after I changed the type locally and I needed to add special checks to make sure the received path is actually `Path`.
  
  Although the change is pretty safe, with listener v2 it's anyway safer to keep using `str`. We are also otherwise just using base types with them.