Fix Issue #21336: Defeault behavior for resolution property in Timestamps
- [x] closes #21336 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Looks good overall! Looks like the CI failed due to some linting errors.  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21365?src=pr&el=h1) Report
> Merging [#21365](https://codecov.io/gh/pandas-dev/pandas/pull/21365?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/92e9882d723c57b3135b17bded1a6495fec0ebda?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21365/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21365?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21365      +/-   ##
==========================================
- Coverage   91.89%   91.89%   -0.01%     
==========================================
  Files         153      153              
  Lines       49600    49596       -4     
==========================================
- Hits        45580    45576       -4     
  Misses       4020     4020
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.29% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.86% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21365?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/21365/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.61% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/21365/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.22% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21365/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.55% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/21365/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `97.03% <0%> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21365?src=pr&el=footer). Last update [92e9882...f28dc5d](https://codecov.io/gh/pandas-dev/pandas/pull/21365?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @mroeschke This passed linting on my local machine, but now there is a simple conflict to resolve under what's new that I can't do because I don't have repo write access. Is this something you can resolve, or point me to someone who can? Thanks. It's possible to fix the merge conflict locally and then re-push the resolution back to this branch. The general flow instructions can be found here: https://pandas.pydata.org/pandas-docs/stable/contributing.html#updating-your-pull-request

Once you execute the `git merge upstream/master` command in your console, you should be prompted to fix the merge conflict with the whatsnew document. Then you can commit the change and push back to this branch. LGTM.  Only nitpick would be with the wording "native pandas", but I don't have any better ideas. Could I get a resolution on this from a contributor? Happy to fiddle the PR if necessary but don't want it to get stale. Hmm looks like this branch picked up the commits of your other PR #21444. You'll need to revert the HEAD of this branch to a prior commit.  can you rebase Looks like #21861 resolved this original issue (#21336). Would still greatly appreciate #21444!