Fix sphinx build: Set django settings to swh.deposit's

Build prior to swh.web module.  It's a workaround the time to find a
more sustainable solution.

Related T763

docs/conf.py

-
 import django
 import os
-os.environ.setdefault("DJANGO_SETTINGS_MODULE", "swh.web.settings.development")
+
+os.environ.setdefault('DJANGO_SETTINGS_MODULE',
+                      'swh.deposit.settings.development')
 django.setup()
 
 from swh.docs.sphinx.conf import *  # NoQA