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>
Apparently, Clubhouse has two different payloads for story label
changes, one where the label name lives inside the "references"
object, and the other where it lives inside the "actions" object.
Their webhook API is still in beta, so this could just be a bug.
Anyhow, we should support both.