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.
From the line in tools/provision it should trickle to the rest of the
scripts. This works since almost all the python scripts have been linted
to be generic.
Proof that the setup is python3 only: with this commit, within the
vagrant container env, /srv/zulip-venv is no longer present and
`./tools/run-dev.py` runs just fine.
[gnprice: Added `rm -f` and warning message, and made small edits.]
Factor out the code in tools/provision.py which installs a python2
and python3 venv into a module (tools/setup/setup_venvs.py) which
can also be used as a script.
