Update dateutil to point at the right mirror.
Update dateutil to point at the right mirror.

TBR=iannucci@chromium.org
BUG=393015

Review URL: https://codereview.chromium.org/389433003

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra