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
