Fix Review Bot extension unit tests on Review Board 3.
Review Request #12454 — Created July 11, 2022 and submitted
The unit tests for Review Bot were unconditionally importing
from the modern
django.urls, which wasn't available on Django 1.6 /
Review Board 3.
They're now updated to fall back on the older import from
django.core.urlresolvers, allowing tests to run on Review Board 3.
Unit tests pass on Review Board 3, 4, and 5.