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>
This experimental setting disables sending private messages in Zulip
in a crude way (i.e. users get an error when they try to send one).
It makes no effort to adjust the UI to avoid advertising the idea of
sending private messages.
Fixes#6617.