[XML-SIG] iso8601: re-creating an original date II

[Andrew Kuchling]

I'm about halfway through my proposed course of enhancing iso8601.py to note
which portions of a date were supplied, but clearly it's reinventing the
wheel. The ISO8601Date class needs a converter to and from 9-tuples, seconds
until the epoch, and string format, and it all feels like I'm reimplementing
the C library or mxDateTime -- badly -- so I'm abandoning the effort.
mxDateTime already includes an ISO-8601 class; from the docs it doesn't seem
to support round-tripping, but that could be added, and it's probably less
work than recreating lots of complicated time handling code.

--amk