Add python_requires to help pip (#529)

Signed-off-by: Hugo <hugovk@users.noreply.github.com>
Signed-off-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

Author: hugovk

setup.py

@@ -31,6 +31,7 @@
         'twisted': ['twisted'],
     },
     test_suite="tests",
+    python_requires=">=2.6, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*",
     classifiers=[
         "Development Status :: 4 - Beta",
         "Intended Audience :: Developers",