Replace `os.path` with `pathlib`

[trag1c]

pathlib offers a simpler and more human-friendly way of dealing with paths and it seems like interrogate mostly uses os.path.

I believe changing this would be more in line with CONTRIBUTING.md :wink:

interrogate tries to take advantage of modern features whenever possible

I'd be happy to work on this if you find this issue reasonable :)

[econchick]

Hey @trag1c - certainly would welcome a patch for this. I can't remember why os.path was originally chosen, it may have been something to do with Windows machines. Or I might be making it up. Either way, so long as it works on Windows + Unix, I'd appreciate it!

[trag1c]

Roger that! Though I think I'll wait for #178 (where I've left a few tips ^^) to merge so that it's easier for me to make the transition if that's okay 😄

[econchick]

#178 is all set (which you've probably seen) - sorry for the lag!