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.
It seems that we no longer get the message, 'zerver/lib/actions.py
modified; restarting server', but the server reloads successfully
nonetheless.
Fixes: #1341.