fix types, document

Author: khvn26

settings/dev.py

@@ -1,7 +1,9 @@
+# Settings expected by `mypy_django_plugin`
 SENDGRID_API_KEY: str
 AWS_SES_REGION_ENDPOINT: str
 SEGMENT_RULES_CONDITIONS_LIMIT: int
 
+# Settings required for tests
 INSTALLED_APPS = [
     "django.contrib.auth",
     "django.contrib.contenttypes",