avoid exceptiosn when not being called by CLI

lib/ansible/plugins/callback/slack.py

@@ -23,7 +23,10 @@ import json
 import os
 import uuid
 
-from __main__ import cli
+try:
+    from __main__ import cli
+except ImportError:
+    cli = None
 
 from ansible.constants import mk_boolean
 from ansible.module_utils.urls import open_url