GHSA-qg36-9jxh-fj25: Incorrect signature verification in django-ses

The django_ses library implements a mail backend for Django using AWS Simple Email Service. The library exports the SESEventWebhookView class intended to receive signed requests from AWS to handle email bounces, subscriptions, etc. These requests are signed by AWS and are verified by django_ses, however the verification of this signature was found to be flawed as it allowed users to specify arbitrary public certificates.