What's the problem this feature will solve?
Retrieving a project's users is not possible via the JSON API. This isn't secret information though, because that information can be scraped from the project's HTML page. That's pretty inconvenient though, requiring tools like BeautifulSoup and tricky parsing of deep HTML documents. It would be so much better if the information were simply available in the "info" dict of the project's JSON API.

Describe the solution you'd like
Add a "users" key to the "info" dictionary of the project's JSON API. Why "users"? The HTML page calls them "maintainers", however the JSON dict already has fields for "author", "author_email", "maintainer", and "maintainer_email". Plus, if you look at the view for the project HTML page, they access the project's .users field. So this seems like the most minimal addition that still makes sense.

Additional context
This is related to #15769

I have a branch I'll be pushing soon.