GitHubMixin.add_label should make it's requests via Twisted's asynchronous requests library.
<rdar://problem/105023225>
Pull request: https://github.com/WebKit/WebKit/pull/9629
Committed 262087@main (075303e0b2dd): <https://commits.webkit.org/262087@main> Reviewed commits have been landed. Closing PR #9629 and removing active labels.