Return start/stop datetime for the quarter as defined by dt.

def quarter(dt):
    """
    Return start/stop datetime for the quarter as defined by dt.
    """
    quarters = rrule.rrule(
       rrule.MONTHLY,
       bymonth = (1, 4, 7, 10),
       bysetpos = -1,
       dtstart = datetime(dt.year, 1, 1),
       count = 8
    )
    first_day = quarters.before(dt, True)
    last_day = quarters.after(dt) - relativedelta.relativedelta(days=1)
    return (first_day, last_day)