Security fix
This fixes a low severity security issue affecting Anymail v0.2–v1.3. (CVE-2018-1000089)
Django error reporting includes the value of your Anymail WEBHOOK_AUTHORIZATION setting. In a properly-configured deployment, this should not be cause for concern. But if you have somehow exposed your Django error reports (e.g., by mis-deploying with DEBUG=True or by sending error reports through insecure channels), anyone who gains access to those reports could discover your webhook shared secret. An attacker could use this to post fabricated or malicious Anymail tracking/inbound events to your app, if you are using those Anymail features.
The fix renames Anymail's webhook shared secret setting so that Django's error reporting mechanism will sanitize it.
If you are using Anymail's event tracking and/or inbound webhooks, you should upgrade to this release and change "WEBHOOK_AUTHORIZATION" to "WEBHOOK_SECRET" in the ANYMAIL section of your settings.py. You may also want to rotate the shared secret value, particularly if you have ever exposed your Django error reports to untrusted individuals.
If you are only using Anymail's EmailBackends for sending email and have not set up Anymail's webhooks, this issue does not affect you.
The old WEBHOOK_AUTHORIZATION setting is still allowed in this release, but will issue a system-check warning when running most Django management commands. It will be removed completely in a near-future release, as a breaking change.
Thanks to Charlie DeTar (@yourcelf) for responsibly reporting this security issue through private channels.