Tweaked by tabbott to not remove it from lister.py, linter_lib, and
friends, since those are intended to support both Python 2 and 3
(we're planning to extract them from the repository).
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.
This helps make the Zulip development environment somewhat more robust
to new contributors, since it will give them a nice warning if they
try running any of our development tools outside the Zulip virtualenv.
Fixes#3468.
rht
Greg Price
Aditya Bansal
Raghav Jajodia
Tim Abbott
sinwar
reallyly
Steve Howell