Add python_requires to help pip

Author: hugovk

Diff for: setup.py

@@ -26,6 +26,7 @@
     license="BSD",
     packages=find_packages(),
     zip_safe=True,
+    python_requires=">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*",
     classifiers=[
         # Picked from
         #    http://pypi.python.org/pypi?:action=list_classifiers