Deployments whose name is not in the format of a timestamp are
always included in the `recent_deployments` and are not deleted,
hence we don't need to check for them explicitly.
The recent rewrite of purge-old-deployments accidentally attempted to
purge the symlinks, sockets, lock, and other files in the deployment
directory.
The new version has been tested out in production successfully.
Expands `purge-old-deployments` such that now it accepts the threshold
days as argument. Also `clean-unused-caches` script is automatically
run after purging the old deployments so that the orphaned caches
gets automatically cleaned.
Fixes: #5726.
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.
