Incorporate "Use os.path.isfile() and isdir()" from freeipa upstream

b29db07c3b3d8937f53684fdbba985fec525d69d by Christian Heimes

    Replace custom file_exists() and dir_exists() functions with proper
    functions from Python's stdlib.

    The change also gets rid of pylint's invalid bad-python3-import error,
    https://github.com/PyCQA/pylint/issues/1565