`test_bucket_w_retention_period` flakes due to EC

[tseaver]

From this Kokoro failure:

________________________ test_bucket_w_retention_period ________________________

storage_client = <google.cloud.storage.client.Client object at 0x7f229517cd10>
buckets_to_delete = [<Bucket: w-retention-period-1629442620182>]
blobs_to_delete = [<Blob: w-retention-period-1629442620182, test-blob, 1629442620932109>]

    def test_bucket_w_retention_period(
        storage_client, buckets_to_delete, blobs_to_delete,
    ):
        period_secs = 10
        bucket_name = _helpers.unique_name("w-retention-period")
        bucket = _helpers.retry_429_503(storage_client.create_bucket)(bucket_name)
        buckets_to_delete.append(bucket)

        bucket.retention_period = period_secs
        bucket.default_event_based_hold = False
        bucket.patch()

        assert bucket.retention_period == period_secs
        assert isinstance(bucket.retention_policy_effective_time, datetime.datetime)
        assert not bucket.default_event_based_hold
        assert not bucket.retention_policy_locked

        blob_name = "test-blob"
        payload = b"DEADBEEF"
        blob = bucket.blob(blob_name)
        blob.upload_from_string(payload)

        blobs_to_delete.append(blob)

        other = bucket.get_blob(blob_name)

        assert not other.event_based_hold
        assert not other.temporary_hold
        assert isinstance(other.retention_expiration_time, datetime.datetime)

        with pytest.raises(exceptions.Forbidden):
            other.delete()

        bucket.retention_period = None
        bucket.patch()

        assert bucket.retention_period is None
        assert bucket.retention_policy_effective_time is None
        assert not bucket.default_event_based_hold
        assert not bucket.retention_policy_locked

        other.reload()

        assert not other.event_based_hold
        assert not other.temporary_hold
>       assert other.retention_expiration_time is None
E       assert datetime.datetime(2021, 8, 20, 6, 57, 10, 949000, tzinfo=<UTC>) is None
E        +  where datetime.datetime(2021, 8, 20, 6, 57, 10, 949000, tzinfo=<UTC>) = <Blob: w-retention-period-1629442620182, test-blob, 1629442620932109>.retention_expiration_time

tests/system/test_bucket.py:572: AssertionError