tools: Create more consistent checks for venv.

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.

tools/diagnose

@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 
 from __future__ import print_function
+from __future__ import absolute_import
 import os
 import platform
 import sys