Add support for Postmark's recently-released [delivery tracking webhook] to Anymail's normailized status event handling. The existing Anymail tracking webhook URL can be copied to "Delivery webhook" in your Postmark outbound server settings.
Closes#45.
Compatibility with Python 2.7 versions older than 2.7.7
* Use Django's constant_time_compare method
* Include sparkpost in test requirements
* Don't use non-public `EnvironmentVarGuard` in tests
Fixes#41
* csrf_exempt must be applied to View.dispatch,
not View.post.
* In base WebhookTestCase, enable Django test Client
enforce_csrf_checks. (Test Client by default disables
CSRF protection.)
Closes#19
