config: Remove Django base class from ZulipSettingsError.

This breaks an import cycle that prevented django-stubs from inferring
types for django.conf.settings.

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

zproject/config.py

@@ -2,10 +2,8 @@ import configparser
 import os
 from typing import Optional, overload
 
-from django.core.exceptions import ImproperlyConfigured
 
-
-class ZulipSettingsError(ImproperlyConfigured):
+class ZulipSettingsError(Exception):
     pass