setup.py: Use setuptools when invoked with bdist_{egg,wheel}.

This makes it easier to build eggs and wheels of 0.19.x releases; the
master branch already uses setuptools.

setup.py

-from distutils.core import setup, Extension
+import sys
+if 'bdist_egg' in sys.argv or 'bdist_wheel' in sys.argv:
+    from setuptools import setup, Extension
+else:
+    from distutils.core import setup, Extension
 from distutils.sysconfig import get_python_lib
 import os, os.path
-import sys
 
 try:
     import platform