[SPARK-12268][PYSPARK] Make pyspark shell pythonstartup work under python3

This replaces the `execfile` used for running custom python shell scripts
with explicit open, compile and exec (as recommended by 2to3). The reason
for this change is to make the pythonstartup option compatible with python3.

Author: Erik Selin <erik.selin@gmail.com>

Closes #10255 from tyro89/pythonstartup-python3.

python/pyspark/shell.py

@@ -76,4 +76,6 @@ if add_files is not None:
 # 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)