Add ml.functions stub with vector_to_array (#317)

* Add ml.functions stub with vector_to_array

* Add since annotation

third_party/3/pyspark/__init__.pyi

@@ -1,6 +1,8 @@
 # Stubs for pyspark (Python 3)
 #
 
+from typing import Callable, TypeVar
+
 from pyspark.status import *
 from pyspark.accumulators import Accumulator as Accumulator, AccumulatorParam as AccumulatorParam
 from pyspark.broadcast import Broadcast as Broadcast
@@ -17,6 +19,10 @@ from pyspark.taskcontext import BarrierTaskContext as BarrierTaskContext, Barrie
 # Compatiblity imports
 from pyspark.sql import SQLContext, Row
 
+T = TypeVar("T")
+
+def since(version: str) -> Callable[[T], T]: ...
+
 # Names in __all__ with no definition:
 #   SparkJobInfo
 #   SparkStageInfo

third_party/3/pyspark/ml/functions.pyi

@@ -0,0 +1,4 @@
+from pyspark import SparkContext as SparkContext, since as since
+from pyspark.sql.column import Column as Column
+
+def vector_to_array(col: Column) -> Column: ...