[pyos][bug] Crossref API requests shoudl have `mailto` and a User Agent that identifies the package

[sneakers-the-rat]

Part of: pyOpenSci/software-submission#267 (comment)

See the crossref api docs: https://www.crossref.org/documentation/retrieve-metadata/rest-api/access-and-authentication/

Users of the crossref API should identify themselves. this also increases the rate and concurrency limits.