Make DatetimeIndex iterator pickleable by dill
Currently, dill (https://github.com/uqfoundation/dill) cannot pickle iterators over DatetimeIndex because they are generators. This simple change removes that limitation.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@rmihael : Thanks for the PR!  The change is relatively harmless it seems, so I'm okay with merging.  However, should get feedback from @jreback and @jorisvandenbossche to double check. yep looks fine. thanks. thanks! @rmihael do you know the reason / case that this was useful? @rmihael to give some context, we reverted this PR in https://github.com/pandas-dev/pandas/pull/21027 because it introduced a bug. Sorry for the silence. I used dill to run multiprocess optimizations using the backtrader framework. I can use cloudpickle with the same success, so there is no need to add extra complexity to the codebase. no problem, thanks for answering!