`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