Fix Timestamp.round errors
- [x] closes #22591
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
- [x] rebase for the new `ambiguous` kwarg, resolve conflicts.
Hello @miccoli! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/indexes/datetimelike.py`](https://github.com/miccoli/pandas/blob/a81b402e242dbf703cdaa04666a79d001d8fc387/pandas/core/indexes/datetimelike.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/datetimes/test_scalar_compat.py`](https://github.com/miccoli/pandas/blob/a81b402e242dbf703cdaa04666a79d001d8fc387/pandas/tests/indexes/datetimes/test_scalar_compat.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/timestamp/test_unary_ops.py`](https://github.com/miccoli/pandas/blob/a81b402e242dbf703cdaa04666a79d001d8fc387/pandas/tests/scalar/timestamp/test_unary_ops.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22802?src=pr&el=h1) Report
> Merging [#22802](https://codecov.io/gh/pandas-dev/pandas/pull/22802?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/302d43a840797175eee04959b8918d3870722a0e?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22802/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22802?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22802      +/-   ##
==========================================
+ Coverage   92.18%   92.19%   +<.01%     
==========================================
  Files         169      169              
  Lines       50821    50827       +6     
==========================================
+ Hits        46851    46860       +9     
+ Misses       3970     3967       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <100%> (ø)` | :arrow_up: |
| #single | `42.37% <33.33%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22802?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/22802/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `98.1% <100%> (ø)` | :arrow_up: |
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/22802/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.63% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/22802/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.45% <0%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/22802/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.2% <0%> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/22802/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.67% <0%> (ø)` | :arrow_up: |
| [pandas/io/formats/style.py](https://codecov.io/gh/pandas-dev/pandas/pull/22802/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvc3R5bGUucHk=) | `96.43% <0%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/22802/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.74% <0%> (ø)` | :arrow_up: |
| [pandas/core/nanops.py](https://codecov.io/gh/pandas-dev/pandas/pull/22802/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvbmFub3BzLnB5) | `95.14% <0%> (+0.01%)` | :arrow_up: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/22802/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.6% <0%> (+0.05%)` | :arrow_up: |
| [pandas/core/internals/managers.py](https://codecov.io/gh/pandas-dev/pandas/pull/22802/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL21hbmFnZXJzLnB5) | `96.66% <0%> (+0.1%)` | :arrow_up: |
| ... and [2 more](https://codecov.io/gh/pandas-dev/pandas/pull/22802/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22802?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/22802?src=pr&el=footer). Last update [302d43a...988e5e3](https://codecov.io/gh/pandas-dev/pandas/pull/22802?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 just merged https://github.com/pandas-dev/pandas/pull/22647, @miccoli will need to rebase to accomadate the ``ambiguous`` kwarg - @mroeschke : I think I addressed your requests 
  1. Nit: 44902695f4adba3406fb4750e819a5776683cae7
  2. `DatetimeIndex` tests: 52329484df7ed099ee56cef86822ced6cc7ae7c3
- @jreback : sorry I had the above two commits ready in my pipeline, but I had not yet read your recent comments. I will address them later.  rebase done the **ci/circleci: py27_compat** failure for 53c8c9b is a false negative, due to a networking problem during the build phase. hope everything is ready for merge (green light). lgtm. waiting till CI is un-borked to merge. thanks @miccoli  thanks for merging