Make pyspark shell pythonstartup work under python3

This replaces the execfile used for running custom python shell scripts
with explicit open, compile and exec. The reason is to make the
pythonstartup option compatible with python3.

python/pyspark/shell.py

@@ -76,4 +76,6 @@
 # which allows us to execute the user's PYTHONSTARTUP file:
 _pythonstartup = os.environ.get('OLD_PYTHONSTARTUP')
 if _pythonstartup and os.path.isfile(_pythonstartup):
-    execfile(_pythonstartup)
+    with open(_pythonstartup) as f:
+        code = compile(f.read(), _pythonstartup, 'exec')
+        exec(code)