Use pathlib instead of os

[jvshields]

I think tardis is trying to migrate over to pathlib instead of using os.path.
This is just an issue to keep track of that and remind ourselves of it for somebody to do at some point.

[jvshields]

I'm fixing this for conftest in PR #386, but it's not great because deeper down the paths are operated on as strings, so you have to convert them back from paths. Someone should look into this in the future.