PCollection boundedness should be tracked and propagated in python sdk

As far as I can tell Python does not care about boundedness of PCollections even in streaming mode, but external transforms do.  In my ongoing effort to get PubsubIO external transforms working I discovered that I could not generate an unbounded write. 

My pipeline looks like this:

    (
        pipe
        | 'PubSubInflow' >> external.pubsub.ReadFromPubSub(subscription=subscription, with_attributes=True)
        | 'PubSubOutflow' >> external.pubsub.WriteToPubSub(topic=OUTPUT_TOPIC, with_attributes=True)
    )

The PCollections returned from the external Read are Unbounded, as expected, but python is responsible for creating the intermediate PCollection, which is always Bounded, and thus external Write is always Bounded.