[SPARK-15803] [PYSPARK] Support with statement syntax for SparkSession

## What changes were proposed in this pull request?

Support with statement syntax for SparkSession in pyspark

## How was this patch tested?

Manually verify it. Although I can add unit test for it, it would affect other unit test because the SparkContext is stopped after the with statement.

Author: Jeff Zhang <zjffdu@apache.org>

Closes #13541 from zjffdu/SPARK-15803.

python/pyspark/sql/session.py

@@ -581,6 +581,22 @@ class SparkSession(object):
         """
         self._sc.stop()
 
+    @since(2.0)
+    def __enter__(self):
+        """
+        Enable 'with SparkSession.builder.(...).getOrCreate() as session: app' syntax.
+        """
+        return self
+
+    @since(2.0)
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        """
+        Enable 'with SparkSession.builder.(...).getOrCreate() as session: app' syntax.
+
+        Specifically stop the SparkSession on exit of the with block.
+        """
+        self.stop()
+
 
 def _test():
     import os