This could be happening in some app that you have pip installed (or equivalent) and
are using in installed apps, or which is being included by some app that is.
You could find-grep in your virtualenv's site-packages for 'default' and pass the result
into grep 'urls', and if necessary, into grep 'conf'.
Another approach is to stick in a set_trace() where the warning is getting triggered or
(since this is presumably your only deprecation warning) where it is getting printed,
and 'u' your way up the stack to see who's importing it, and, if necessary, what they
are being imported, etc.
Having found the source, you could then:
See if there is a newer version with it fixed
Patch it yourself (and submit the patch to the maintainer)
Or live with it -- for now it's only a deprecation warning. You will have to find it if you
upgrade Django to the point that it fails.
Bill