mypy: Added Dict, List and Set imports.

Fixed mypy errors associated with the upgrade.

zerver/webhooks/greenhouse/view.py

@@ -2,7 +2,7 @@ from __future__ import absolute_import
 
 from django.utils.translation import ugettext as _
 from django.http import HttpRequest, HttpResponse
-from typing import Any
+from typing import Any, Dict, List
 
 from zerver.lib.actions import check_send_message
 from zerver.lib.response import json_success, json_error