www/py-django-simple-captcha: Fix build with DOCS option enabled

running build_sphinx
Running Sphinx v5.3.0

Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):

File "/usr/local/lib/python3.9/site-packages/sphinx/config.py", line 350, in eval_config_file
  exec(code, namespace)
File "/wrkdirs/usr/ports/www/py-django-simple-captcha/work-py39/django-simple-captcha-0.5.14/docs/conf.py", line 17, in <module>
  from six import u

ModuleNotFoundError: No module named 'six'

• Error code 1

Approved by: portmgr (blanket)
With hat: python