code: func (s *PullRequestsService) ListReviewers(ctx context.Context, owner, repo string, number int, opt *ListOptions) (*Reviewers, *Response, error) {
  	u := fmt.Sprintf("repos/%v/%v/pulls/%d/requested_reviewers", owner, repo, number)
  	u, err := addOptions(u, opt)
  	if err != nil {
  		return nil, nil, err
  	}
  
  	req, err := s.client.NewRequest("GET", u, nil)
  	if err != nil {
  		return nil, nil, err
  	}
  
  	reviewers := new(Reviewers)
  	resp, err := s.client.Do(ctx, req, reviewers)
  	if err != nil {
  		return nil, resp, err
  	}
  
  	return reviewers, resp, nil
  }
docs: // ListReviewers lists reviewers whose reviews have been requested on the specified pull request.
  //
  // GitHub API docs: https://developer.github.com/v3/pulls/review_requests/#list-review-requests