Log Message:
py-six: updated to 1.13.0

1.13.0
- Add `six.moves.dbm_ndbm`.
- Add `six.moves.collections_abc`, which aliases the `collections`
  module on Python 2-3.2 and the `collections.abc` on Python 3.3 and greater.
- Re-add distutils fallback in `setup.py`.
- On Python 3.7, `with_metaclass` supports classes using PEP
  560 features.