internal: Fix a severe bug with the file-system wrappers on OSX. `os.walk` is expected to support modifying its return values `dirnames` and `filenames` so that you can filter out items that need to be iterated upon. I was previously returning a different list in the wrappers, which means filtering was completely broken.

import os


def slashfix(path):
    if path is None:
        return None
    if isinstance(path, str):
        return path.replace('/', os.sep)
    fixed = []
    for p in path:
        fixed.append(p.replace('/', os.sep))
    return fixed