Change paths passed to listener v3 methods to pathlib.Path instances

[pekkaklarck]

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 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.