BUG:Floating point accuracy with DatetimeIndex.round (#14440)
 - [x] closes #14440
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

Employs @eoincondron's fix for float point inaccuracies when rounding by milliseconds for `DatetimeIndex.round` and `Timestamp.round`  