Force distutils to avoid depending on setuptools

distutils and setuptools have differing behavior in regard to egg-info
directories versus files, so we need to ensure that no automagic behavior
occurs here.

We cannot hard-depend on setuptools due to a circular dependency.

diff --git a/setup.py b/setup.py
--- a/setup.py
+++ b/setup.py
@@ -1,9 +1,6 @@
 from __future__ import with_statement
 
-try:
-    from setuptools import setup
-except ImportError:
-    from distutils.core import setup
+from distutils.core import setup
 
 import six