The Github API docs for PullRequests say to make use of the Issues API
to add, remove, set and query labels. This means that we need to make
use of the issue_url property for the PullRequest, not its own URL.
PullRequests can also have labels attached to them, like Issues or
Milestones. Extend PullRequest to also export them if they are present,
as well as adding methods to add, remove and set labels.
MarcoFalke