Regression in __getitem__ raising for slice DatetimeIndex
- [x] closes #43223
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

@jbrockmendel Any idea why we used ``maybe_indices_to_slice`` instead of take like in loc?
