Verbose print causes pipelines to get stuck on FlinkRunner.

Minimal reproduction: 
 
def _run_pipeline(pipeline):
    def process_data(unused):
        print('a'*1000)

    _ = (
            pipeline
            | "Create" >> beam.Create(['a']*1000)
            | "Process" >> beam.Map(process_data)
    )
    pipeline.run().wait_until_finish()

See thread at https://lists.apache.org/thread/r971f432mvko4g3hmnrodh8s4n2wkk6p