[BUG] Cloudpickle is unnecessarily a hard dependency of Spark backend

[goodwanghan]

This change: adf3884#diff-f6002ac0db7dcceed0d45e617d3c625008cff4d1807cf64a4702821cd2aa5d17R3 introduced hard dependency of cloudpickle to fugue spark backend. And from pyspark 3.4.0, cloudpickle is no longer a dependency of pyspark.

So when users install fugue[spark] in a spark 3.4 environment, fugue may complain that the spark objects are not recognized, it is because without cloudpickle, fugue_spark could not be correctly imported.

The solution is to change cloudpickle to pickle for the specific usage inside fugue_spark.