Add a python-dateutil dependency to Djblets 1.x.
Review Request #9931 — Created May 11, 2018 and submitted
This adds a dependency on python-dateutil to Djblets 1.x. We now need
this here for consent tracking. We already include a dependency on
python-dateutil (though a newer version) on Djblets 2.x.
This is the same version used by Review Board, helping packagers keep
./setup.py developand verified it was fetching python-dateutil.