Pass python_requires argument to setuptools (#224)

Helps pip decide what version of the library to install.

https://packaging.python.org/tutorials/distributing-packages/#python-requires

> If your project only runs on certain Python versions, setting the
> python_requires argument to the appropriate PEP 440 version specifier
> string will prevent pip from installing the project on other Python
> versions.

https://setuptools.readthedocs.io/en/latest/setuptools.html#new-and-changed-setup-keywords

> python_requires
>
> A string corresponding to a version specifier (as defined in PEP 440)
> for the Python version, used to specify the Requires-Python defined in
> PEP 345.
diff --git a/setup.py b/setup.py
index 72b70c9..384a2e8 100644
--- a/setup.py
+++ b/setup.py
@@ -53,5 +53,6 @@
       description="Python 2 and 3 compatibility utilities",
       long_description=six_long_description,
       license="MIT",
-      classifiers=six_classifiers
+      classifiers=six_classifiers,
+      python_requires=">=2.6, !=3.0.*, !=3.1.*",
       )