Use "python_requires" for setuptools and depend on Python 2.7+ <3

setup.py

@@ -37,6 +37,7 @@ setup(
     author_email=__email__,
     url=__url__,
     license=__license__,
+    python_requires='>=2.7, <3',
     install_requires=install_requires,
     packages=find_packages(),
     package_data={