caos.backends.pgsql: Fix DateTime packing
caos.backends.pgsql: Fix DateTime packing

py-postgresql packing agorithm includes transformation of the DateTime
value into an absolute time interval, which for Semantix DateTime is
represented by TimeDelta, which is not compatible with
datetime.timedelta.  The latter stores the delta as days, seconds,
microseconds triple, which py-postgresql expects to see, whereas
TimeDelta stores normalized values.  This resulted in hour/minute
information being lost for datetime atoms on updates.

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