code: func (s *ProjectsService) ListProjectColumns(ctx context.Context, projectID int64, opt *ListOptions) ([]*ProjectColumn, *Response, error) {
  	u := fmt.Sprintf("projects/%v/columns", projectID)
  	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
  	}
  
  	// TODO: remove custom Accept headers when APIs fully launch.
  	req.Header.Set("Accept", mediaTypeProjectsPreview)
  
  	columns := []*ProjectColumn{}
  	resp, err := s.client.Do(ctx, req, &columns)
  	if err != nil {
  		return nil, resp, err
  	}
  
  	return columns, resp, nil
  }
docs: // ListProjectColumns lists the columns of a GitHub Project for a repo.
  //
  // GitHub API docs: https://developer.github.com/v3/projects/columns/#list-project-columns