BUG:Floating point accuracy with DatetimeIndex.round (#14440)
State: closed

 - [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`  

Labels:
name: Bug
name: Datetime
description: Datetime data dtype

Comments:
small testing addition. ping when pushed / green.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15568?src=pr&el=h1) Report
> Merging [#15568](https://codecov.io/gh/pandas-dev/pandas/pull/15568?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d6524850c8dea36ab37536e439999fd121b95429?src=pr&el=desc) will **decrease** coverage by `-0.03%`.
> The diff coverage is `100%`.


```diff
@@            Coverage Diff             @@
##           master   #15568      +/-   ##
==========================================
- Coverage   91.05%   91.03%   -0.03%     
==========================================
  Files         137      137              
  Lines       49291    49291              
==========================================
- Hits        44883    44873      -10     
- Misses       4408     4418      +10
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15568?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/base.py](https://codecov.io/gh/pandas-dev/pandas/compare/d6524850c8dea36ab37536e439999fd121b95429...c5a7cbc5035860ef49347e5c2464daf95a2c0c50?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvYmFzZS5weQ==) | `96.59% <100%> (ø)` | :white_check_mark: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/d6524850c8dea36ab37536e439999fd121b95429...c5a7cbc5035860ef49347e5c2464daf95a2c0c50?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :x: |
| [pandas/tools/merge.py](https://codecov.io/gh/pandas-dev/pandas/compare/d6524850c8dea36ab37536e439999fd121b95429...c5a7cbc5035860ef49347e5c2464daf95a2c0c50?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL21lcmdlLnB5) | `91.78% <0%> (-0.35%)` | :x: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/d6524850c8dea36ab37536e439999fd121b95429...c5a7cbc5035860ef49347e5c2464daf95a2c0c50?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.83% <0%> (-0.1%)` | :x: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/d6524850c8dea36ab37536e439999fd121b95429...c5a7cbc5035860ef49347e5c2464daf95a2c0c50?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.36% <0%> (+0.33%)` | :white_check_mark: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15568?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15568?src=pr&el=footer). Last update [d652485...c5a7cbc](https://codecov.io/gh/pandas-dev/pandas/compare/d6524850c8dea36ab37536e439999fd121b95429...c5a7cbc5035860ef49347e5c2464daf95a2c0c50?src=pr&el=footer&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
thanks!