Add python_requires to help pip

Author: hugovk

setup.py

@@ -161,6 +161,7 @@
             ],
     },
     install_requires=['PyOpenSSL', 'pyasn1>=0.1.1'],
+    python_requires='>=2.6, !=3.0.*, !=3.1.*, !=3.2.*',
     classifiers=[
         'Development Status :: 5 - Production/Stable',
         'Environment :: Console',