Use setuptools when available

This is necessary for providing wheels.  See #3.

Author: ddfisher

setup.py

@@ -1,4 +1,7 @@
-from distutils.core import setup, Extension
+try:
+    from setuptools import setup, Extension
+except ImportError:
+    from distutils.core import setup, Extension
 
 _ast27 = Extension(
     '_ast27',