Bail if building with Py3

Author: Jordan Adler

setup.py

@@ -12,6 +12,9 @@
 except ImportError:
     from distutils.core import setup
 
+if sys.argv[-1] == 'sdist' and sys.version_info[0] >= 3:
+    print('ERROR: You must build python-future with Python 2', file=sys.stderr)
+    sys.exit(1)
 
 if sys.argv[-1] == 'publish':
     os.system('python setup.py sdist upload')