Add a warning if trying to run on anything but Python 2.7

----Release Notes----
[]
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=121849170

Author: silviulica

google/cloud/dataflow/__init__.py

@@ -60,6 +60,15 @@
 """
 
 
+import sys
+
+
+if sys.version_info.major != 2:
+  raise RuntimeError(
+      'Dataflow SDK for Python is supported only on Python 2.7. '
+      'It is not supported on Python [%s].' % sys.version)
+
+
 import google.cloud.dataflow.internal.pickler
 
 from google.cloud.dataflow import coders