docs: def _handle_exception():
      """Print exceptions raised by subscribers to stderr."""
      # Heavily influenced by logging.Handler.handleError.
  
      # See note here:
      # https://docs.python.org/3.4/library/sys.html#sys.__stderr__
      if sys.stderr:
          einfo = sys.exc_info()
          try:
              traceback.print_exception(einfo[0], einfo[1], einfo[2],
                                        None, sys.stderr)
          except IOError:
              pass
          finally:
              del einfo