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

def quarter(d):
    """
    Return start/stop datetime for the quarter as defined by dt.
    """
    from django_toolkit.datetime_util import quarter as datetime_quarter
    first_date, last_date = datetime_quarter(datetime(d.year, d.month, d.day))
    return first_date.date(), last_date.date()