test_events: Add missing print_function py3 import.

zerver/tests/test_events.py

@@ -2,6 +2,7 @@
 # See http://zulip.readthedocs.io/en/latest/events-system.html for
 # high-level documentation on how this system works.
 from __future__ import absolute_import
+from __future__ import print_function
 from typing import Any, Callable, Optional
 
 from django.conf import settings