Update ecosystem.rst to include Pint
We are working on upgrading pint to be compatible with pandas, see https://github.com/hgrecco/pint/pull/684

I am guessing that the line in the docs,

> If you’re building a library that implements the interface, please publicize it on Extension Data Types.

meant something like this pull request. If that's completely wrong, apologies.

- [x] closes #xxxx (N/A as not directly related to an issue, but makes progress towards #10349 )
- [x] (N/A) tests added / passed 
- [x] (N/A) passes `git diff upstream/master -u -- "*.py" | flake8 --diff` *
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22582?src=pr&el=h1) Report
> Merging [#22582](https://codecov.io/gh/pandas-dev/pandas/pull/22582?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e5c90e5e355077c4ee5245a4a72833f4e8af72b9?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22582/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22582?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22582   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files         161      161           
  Lines       51497    51497           
=======================================
  Hits        47530    47530           
  Misses       3967     3967
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.69% <ø> (ø)` | :arrow_up: |
| #single | `42.42% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22582?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/22582?src=pr&el=footer). Last update [e5c90e5...93dd398](https://codecov.io/gh/pandas-dev/pandas/pull/22582?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I’ll take a look at pint#684; units support have been an often-requested feature for pandas. We know the version number from https://github.com/hgrecco/pint/issues/693#issuecomment-419212581, can I do anything else? @jreback @jbrockmendel any idea why this is failing..? > any idea why this is failing..?

Looks like a CI bug that was cleared up a couple days ago.  Try rebasing? can you merge master and update We're working on a new release of Pint (https://github.com/hgrecco/pint/issues/693) but none of us can find the time right now, I hope to in January CI is green now, is this ready to be merged, or are we waiting for the next Pint release? @znicholls is this ready to be merged? or is a new Pint release or something else needed? > or is a new Pint release or something else needed?

Up to you guys. It will be included in the next Pint release but maybe best wait given we're still a bit uncertain about how best to do it (Panda's pytest test suite isn't playing nice with Pint's unittest test suite and we don't understand why, see https://github.com/hgrecco/pint/pull/741) Ok, I see. Thanks for the clarification. I'll then close this PR for now, but please ping us when you update Pint, so we reopen it and discuss any needed change then. @datapythonista we have release pint-pandas on pypi (https://pypi.org/project/Pint-Pandas/). I've re-written the docs (see e.g. https://github.com/znicholls/pandas/blob/patch-1/doc/source/ecosystem.rst#pint-pandas) so can we re-open or make a new PR please? 

FYI @hgrecco and @andrewgsavage GitHub doesn't let me reopen this PR, since the branch has been forced pushed or recreated. Can you open a new PR please?

Also, the ecosystem was moved to the website, it's a different file now.

Thanks for the work on this. ok cool have made #35170 