code: func (s *ProjectsService) ReviewProjectCollaboratorPermission(ctx context.Context, id int64, username string) (*ProjectPermissionLevel, *Response, error) {
  	u := fmt.Sprintf("projects/%v/collaborators/%v/permission", id, username)
  	req, err := s.client.NewRequest("GET", u, nil)
  	if err != nil {
  		return nil, nil, err
  	}
  
  	// TODO: remove custom Accept header when this API fully launches.
  	req.Header.Set("Accept", mediaTypeProjectsPreview)
  
  	ppl := new(ProjectPermissionLevel)
  	resp, err := s.client.Do(ctx, req, ppl)
  	if err != nil {
  		return nil, resp, err
  	}
  	return ppl, resp, nil
  }
docs: // ReviewProjectCollaboratorPermission returns the collaborator's permission level for an organization
  // project. Possible values for the permission key: "admin", "write", "read", "none".
  // You must be an organization owner or a project admin to review a user's permission level.
  //
  // GitHub API docs: https://developer.github.com/v3/projects/collaborators/#review-a-users-permission-level