[pgAdmin4][Patch]: Properly handle builtins.SERVER_MODE variable when SERVER_MODE is not set

Hi

While configuring pgAdmin4 with Apache mod_wsgi, it fails to run apache
server due to python error.

[mod_wsgi (pid=12596): Target WSGI script
'/opt/pgadmin4_2/virtualenv/lib/python2.7/site-packages/pgadmin4/pgAdmin4.wsgi'
cannot be loaded as Python module.
mod_wsgi (pid=12596): Exception occurred processing WSGI script
'/opt/pgadmin4_2/virtualenv/lib/python2.7/site-packages/pgadmin4/pgAdmin4.wsgi'.
Traceback (most recent call last):
File "/opt/pgadmin4_2/virtualenv/lib/python2.7/site-packages/pgadmin4/pgAdmin4.wsgi",
line 20, in <module>
import config
File "/opt/pgadmin4_2/virtualenv/lib/python2.7/site-packages/pgadmin4/config.py",
line 118, in <module>
if builtins.SERVER_MODE is None:
AttributeError: 'module' object has no attribute 'SERVER_MODE'

​Reported by: ​
Gabriel Sánchez

Please find an attached patch.

Thanks,
Surinder
​