Use os.path.isfile() and isdir()
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
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>