Import ImproperlyConfigured

Forgot to import this when adding it in my previous PR.

Author: chigby

wagtail_newsletter/models.py

@@ -1,6 +1,6 @@
 from typing import Any, Optional
 
-from django.core.exceptions import ValidationError
+from django.core.exceptions import ImproperlyConfigured, ValidationError
 from django.db import models
 from django.conf import settings
 from django.http import HttpResponse