The event name seems to have been incorrectly called `todo_due_date_changed`
instead of `todo_due_on_changed`. The API docs for webhooks don't mention
the correct event name, but the TODO json payload[1] seems to contain the
`due_on` field, aside from the fixture actually referring to
`todo_due_on_changed` event type.
[1]: https://github.com/basecamp/bc3-api/blob/master/sections/todos.md
Generated by `pyupgrade --py3-plus --keep-percent-format` on all our
Python code except `zthumbor` and `zulip-ec2-configure-interfaces`,
followed by manual indentation fixes.
Signed-off-by: Anders Kaseorg <anders@zulipchat.com>