[SPARK-2470] PEP8 fixes to shell.py

Author: nchammas

python/pyspark/shell.py

@@ -35,7 +35,8 @@
 from pyspark.storagelevel import StorageLevel
 
 # this is the equivalent of ADD_JARS
-add_files = os.environ.get("ADD_FILES").split(',') if os.environ.get("ADD_FILES") is not None else None
+add_files = (
+    os.environ.get("ADD_FILES").split(',') if os.environ.get("ADD_FILES") is not None else None)
 
 if os.environ.get("SPARK_EXECUTOR_URI"):
     SparkContext.setSystemProperty("spark.executor.uri", os.environ["SPARK_EXECUTOR_URI"])