python: Convert "".format to Python 3.6 f-strings.

Generated by pyupgrade --py36-plus --keep-percent-format, but with the
NamedTuple changes reverted (see commit
ba7906a3c6, #15132).

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/webhooks/splunk/view.py

@@ -31,7 +31,7 @@ def api_splunk_webhook(request: HttpRequest, user_profile: UserProfile,
 
     # for the default topic, use search name but truncate if too long
     if len(search_name) >= MAX_TOPIC_NAME_LENGTH:
-        topic = "{}...".format(search_name[:(MAX_TOPIC_NAME_LENGTH - 3)])
+        topic = f"{search_name[:(MAX_TOPIC_NAME_LENGTH - 3)]}..."
     else:
         topic = search_name