Why pybind11 generated enum type is not PEP 435 compatible. say, it doesn't have value attribute and it's not iterable.

EDIT(eric): Deleted template text in body. Title describes issue sufficiently.

For reference: [PEP 435](https://www.python.org/dev/peps/pep-0435/)

EDIT 2 (eric): See #253 per Ashley's mention.