Accept dateutil frequency constants in date_range
- [x] closes #9314
- [x] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @mason-98! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-03-13 19:31:15 UTC edit: ignore my comment above; overlooked the first module! Thanks for your interest in this PR @mason-98. I think we need more discussion on whether we want to accept this feature in the original issue as the dateutil frequencies are just integers and not enums.

Feel free to tackle other `good first issue` tags if interested! Going to close for now 