Use pathlib.Path instead of os.path

[snukky]

Following #25 (comment), we would like to switch from using os.path to pathlib.Path.

[NovusEdge]

I'd like to take this one up if this issue isn't closed yet.

[cfedermann]

@NovusEdge Hi! If you are still interested, feel free to work on this :) Can you please add a reply to indicate yay or nay? Thanks!

Note that we are currently running submissions for WMT22, so the latest code is in the dev branch, not main.

[NovusEdge]

Please assign the issue to me, so that I can proceed and work on this

[NovusEdge]

@cfedermann, I've made the changes. Please check the PR to see if everything is in order.