Removed code duplicated in SQLite3 and SpatiaLite GeoDjango DB backends.

Moved it to an auxiliary method in the SQLite3 backend cursor class.
Did this to reduce the chances of us forgetting to port changes in DB
connection setup process from the former to the latter one like it
happened e.g. in r17205.

Refs #17258.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17499 bcc190cf-cafb-0310-a4f2-bffc1f526a37