Add dateutil code

Borrowed (with license) from python-dateutil.  We're copying
code here instead of inserting it as a package.  Because
(1) this package is not supported by OS.
(2) all new OS packages must be python 3 compliant.
(3) this package has a python-3 compliant version and a python-2 version
(4) Congress is not yet python-3 compliant

Change-Id: Idaa29ecd9fc6d12298ca441748f71b6e41262d8f