spark.kubernetes.pyspark.pythonVersion is never passed to executors

I suppose this may not be a priority with Python2 on its way out, but given that this setting is only ever sent to the driver and not the executors, no actual work can be performed when the versions don't match, which will tend to be always with the default setting for the driver being changed from 2 to 3, and the executors using `python`, which defaults to v2, by default.