[FIX] stock: use timedelta instead of relativedelta
[FIX] stock: use timedelta instead of relativedelta

Suppose we have a delta between two datetimes:

    delta = dt1 - dt0

If delta is negative but less than a day, ``delta.days`` returns -1 (which is
compensated by positive seconds).  In order to avoid this surprising effect,
use ``delta.total_seconds()`` to compute the number of days.

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