Add support for `requested_teams` to `PullRequest`

[exarkun]

GitHub pull requests have a requested_teams array which is similar to the requested_reviewers array but for reviews requested from teams instead of individual users.

The PullRequest could support this information in a way similar to how it supports requested_reviewers.

[exarkun]

@phadej Just a gentle ping to be sure you saw the linked PR. Thanks.