use pathlib instead of os.path

[iamdefinitelyahuman]

What I did

Replace os.path with pathlib.Path - it makes for much cleaner code and reduces headaches when dealing with Windows file systems.

How to verify it

Run the tests.

Cute Animal Picture

[iamdefinitelyahuman]

Did this as a lead-up to #1367 - implementing relative imports will mean a lot more interaction with the file system.

[davesque]

Thanks, looks much cleaner. Reviewing these changes reminds me of our need for script invocation tests! :)

[davesque]

@fubuloubu @iamdefinitelyahuman By the way, I did run the vyper bin manually a couple of times to make sure the affected code paths were still behaving as expected, but I think I or someone should revisit the script invocation pr #1337 .