Explore use of pathlib instead of os.path

[Pennycook]

Feature/behavior summary

pathlib offers a higher-level abstraction of paths than os.path, including a Path object. Moving to pathlib would make the code cleaner, and would make it simpler for us to expose (via type-hints and docstrings) when certain functions expect paths.

Request attributes

• Would this be a refactor of existing code?
• Does this proposal require new package dependencies?
• Would this change break backwards compatibility?

Related issues

N/A

Solution description

• Migrate existing uses of os.path to pathlib
• Update interfaces to accept Path or os.PathLike instead of str

Additional notes

If applied generally, I think this would break backwards compatibility -- some functions accepting strings should probably accept a Path (or os.PathLike).