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>
"Local" datetimes are local to the server (or rather, are using
settings.TIME_ZONE), which in most cases is not what the recipient of the
message is expecting.