Importing chaco without X server running causes a crash.

[midhun-pm]

Problem Description
Trying to import chaco without X server running on Linux causes python to terminate.

Reproduction Steps:

1. Ensure that the DISPLAY environment variable has been unset in the terminal.
   unset DISPLAY
2. Start the Python3.6 interpreter and run
   import chaco.api

import chaco.api

Expected behavior:
Import should not cause a crash.
This is important for example, when using chaco on a remote server without X server running.
Error on trying to display something is acceptable but not a crash when importing.

OS, Python version: Linux, Python3.6

[midhun-pm]

(esg-automation)midhun@midhun-lnx:~$ unset DISPLAY
(esg-automation)midhun@midhun-lnx:~$ python
Enthought Deployment Manager -- https://www.enthought.com
Python 3.6.0 |Enthought, Inc. (x86_64)| (default, Feb 25 2020, 20:22:55) 
[GCC 4.4.7 20120313 (Red Hat 4.4.7-23)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import chaco.api
: cannot connect to X server 
(esg-automation)midhun@midhun-lnx:~$ 

[jwiggins]

I suspect this is fixed by Enable 5.0.0 since it removes the OpenGL linkage from kiva.agg. It's worth testing.

[jwiggins]

Ref: enthought/enable#392

[corranwebster]

Also perhaps explicitly setting the ETS_TOOLKIT environment variable to null to stop ETS trying to import Qt/Wx.