[di]

You are correct, the classifiers are just search qualifiers.

However, since pip 9.0.0, it will respect the Requires-Python[0] metadata, which can be specified via your setup.py file.

Since PyPI allows only a single source distribution per release, though, this will still not get you separate modules for separate codebases as it seems you originally desired. As mentioned in a previous comment, it's generally preferred to support multiple versions in a single codebase via six.

[0]: https://www.python.org/dev/peps/pep-0345/#requires-python