zulip bridge: Use python instead of python3 in script.

This script moved from the bots package to the API package, and does not
explicitly require python3.

zulip/integrations/bridge_between_zulips/run-interrealm-bridge

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
 # -*- coding: utf-8 -*-
 
 import sys