This causes `upgrade-zulip-from-git`, as well as a no-option run of
`tools/build-release-tarball`, to produce a Zulip install running
Python 3, rather than Python 2. In particular this means that the
virtualenv we create, in which all application code runs, is Python 3.
One shebang line, on `zulip-ec2-configure-interfaces`, explicitly
keeps Python 2, and at least one external ops script, `wal-e`, also
still runs on Python 2. See discussion on the respective previous
commits that made those explicit. There may also be some other
third-party scripts we use, outside of this source tree and running
outside our virtualenv, that still run on Python 2.
ScheduledJob was written for much more generality than it ended up being
used for. Currently it is used by send_future_email, and nothing
else. Tailoring the model to emails in particular will make it easier to do
things like selectively clear emails when people unsubscribe from particular
email types, or seamlessly handle using the same email on multiple realms.
datetime.utcnow() is a timezone-naive datetime. The Django ORM interprets it
in the settings.TIME_ZONE timezone (e.g. 'America/New_York' in the
development server). We perhaps haven't noticed errors yet since with
'America/New_York' all it means is that emails are sent 5 hours early, or a
slightly different set of messages are included in the digest.
