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
