Expose project_urls in JSON API

[nixjdm]

https://packaging.python.org/tutorials/distributing-packages/#project-urls

Related to #3798 / #3820

I realized project_urls is not currently exposed by the JSON API. I propose adding it.

Though the keys in the project_urls dict can be anything, they're fairly standardized, enough to be useful when querying for them over and API. For example, Flask's API response lists its home_page as https://www.palletsprojects.com/p/flask/ (not it's github account which is fairly typical), and puts it's GitHub link in project_urls['Code'], which is not currently in the API response.

[di]

@nixjdm Shall I assign this to you?

[nixjdm]

Sure :) It's so similar it'd be no problem, especially after the other PR get's in.

[tomdottom]

@nixjdm I was wondering how work was going on this feature?

[nixjdm]

I had to take off from doing this for a while, but it's done, with pending review as a pull request #4184!