Throw clear install error on unsupported Python versions

Fixes #7.

Author: ddfisher

setup.py

@@ -1,3 +1,7 @@
+import sys
+if sys.version_info[0] < 3 or sys.version_info[1] < 3:
+    sys.exit('Error: typed_ast only runs on Python 3.3 and above.')
+
 try:
     from setuptools import setup, Extension
 except ImportError: