Right now we're just using the trove classifiers, so I think people may end up doing some complicated pinning, which is less than ideal (see #536 and #526). What we need is the Requires-Python: metadata in pip. I think this is added through setup.py. Here are some examples of people using it:

• pytest
• ipython

@Carreau gave this talk at PyBay (link goes to the relevant time - 11 minutes in) where he discusses the python_requires option (slides are here), probably best to listen to what he's talking about and avoid or document the pitfalls here.