add Juneteenth to USFederalHolidayCalendar
* Add **Juneteenth National Independence Day**

*  Standardize the [official federal holiday names](https://www.opm.gov/policy-data-oversight/pay-leave/federal-holidays/#url=2021)
Can you write a test for this. Hello @ChuliangXiao! 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 2021-07-02 15:35:29 UTC Thanks @ChuliangXiao, besides the test, can you also please an entry in the release notes? Thanks! This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. Closing as stale, but if you want to finish this @ChuliangXiao just let me know.