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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def retention_policy_effective_time(self):
 """Retrieve the effective time of the bucket's retention policy. :rtype: datetime.datetime or ``NoneType`` :returns: point-in time at which the bucket's retention policy is effective, or ``None`` if the property is not set locally. """

        policy = self._properties.get("retentionPolicy")
        if policy is not None:
            timestamp = policy.get("effectiveTime")
            if timestamp is not None:
                return _rfc3339_to_datetime(timestamp)