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.
* Now queue_workers.py sorts queue names and prints them on their own
line. Previously it's output was nondeterministic.
* Simplified grep strategy for removing the "test" worker.
This list was likely to end up out of date quickly, since it wasn't
documented that you need to update it when adding a queue. The best
solution is to just not require it to be updated.