[SPARK-8339] [PYSPARK] integer division for python 3

Itertools islice requires an integer for the stop argument.  Switching to integer division here prevents a ValueError when vs is evaluated above.

davies

This is my original work, and I license it to the project.

Author: Kevin Conor <kevin@discoverybayconsulting.com>

Closes #6794 from kconor/kconor-patch-1 and squashes the following commits:

da5e700 [Kevin Conor] Integer division for batch size

python/pyspark/serializers.py

@@ -272,7 +272,7 @@ class AutoBatchedSerializer(BatchedSerializer):
             if size < best:
                 batch *= 2
             elif size > best * 10 and batch > 1:
-                batch /= 2
+                batch //= 2
 
     def __repr__(self):
         return "AutoBatchedSerializer(%s)" % self.serializer