Drop dependency on python-dateutil

Review Request #9898 — Created May 1, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-4.0.x
26cedde...

Reviewers

As of /r/9922/ Djblets now depends on python-dateutil >= 2.7, which
is sufficient for our purposes and we no longer require an explicit
dependency on it.

Ran unit tests.