I went through each method in the source and verified the inputs it accepts and outputs it gives. I also redefined a few methods on the `Path` class so that MyPy knows they return `Path` instead of `PurePath`. This is really just a temporary workaround until https://github.com/python/mypy/issues/1212 is fixed, but greatly improves the process of working with the `pathlib` module and is therefore likely worth the duplication.
