Requires pyspark >=2.3

[James Colvin]

While the requirements don't state a specific pyspark version is required, the use of the _to_corrected_pandas_type function from pyspark.sql.dataframe requires pyspark 2.3, as that was when that function was introduced.

Either the requirements should be updated, or just copy-paste that function into intake-spark.

[Martin Durant]

Thanks for the note.
Would you be interested in supplying a PR for this change?