IssueTracker

Please provide as much information as possible. At least, this should include a description of your issue and steps to reproduce the problem. If possible please provide a summary of what steps or workarounds you have already tried, and any docs or articles you found (un)helpful.

Problem you have encountered:
When running a batch prediction job it runs using Python 2, in [1] is mentioned that "Python 3.5 is available for online prediction.", but not for batch predictions.

What you expected to happen:
Get the feature to support Python 3.5 for batch predictions.

Steps to reproduce:
If you try to specify the Python version in the request :

body = {
        'jobId': job_id,
        'predictionInput': {
            'dataFormat': data_format,
            'inputPaths': input_paths,
            'outputPath': output_path,
            'region': region,
            'runtimeVersion': '1.14',
            'uri': 'gs://BUCKET/',
            'pythonVersion': '3.5'
        }
    }

Errors like [2] appears and is not possible to run the job, which indicates that is not possible to specify the Python version in the request.

[1] https://cloud.google.com/ml-engine/docs/release-notes#april_5_2018
[2] Invalid JSON payload received. Unknown name "pythonVersion" at 'job.prediction_input': Cannot find field.