docs: def load_backend(backend_name):
      """ load pool backend."""
      try:
          if len(backend_name.split(".")) > 1:
              mod = import_module(backend_name)
          else:
              mod = import_module("spamc.backend_%s" % backend_name)
          return mod
      except ImportError:
          error_msg = "%s isn't a spamc backend" % backend_name
          raise ImportError(error_msg)