botserver: Fix mypy error.
@@ -12,6 +12,7 @@ from werkzeug.exceptions import BadRequest
from six.moves.configparser import SafeConfigParser
from zulip import Client
+from zulip_bots.custom_exceptions import ConfigValidationError
from zulip_bots.lib import ExternalBotHandler, StateHandler
bots_config = {} # type: Dict[str, Mapping[str, str]]
