BUG: DatetimeIndex with nanosecond frequency does not include end
- [x] closes #13672
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13762?src=pr) is 84.56% (diff: 100%)

> Merging [#13762](https://codecov.io/gh/pydata/pandas/pull/13762?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13762   diff @@
==========================================
  Files           141        141          
  Lines         51195      51195          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43295      43295          
  Misses         7900       7900          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [9f94e6a...0e63d70](https://codecov.io/gh/pydata/pandas/compare/9f94e6a04564e8269b43c5b4e83854d277ea30bd...0e63d70845388a422337561802b1f9081a379328?src=pr)
 Looks good to me!
