title: Is there a standard way to declare old python versions unsupported?
body: <p>I want to officially drop python 2 support for a program I maintain and take full advantage of python 3 features. Instead of having a program that more-or-less works under python 2 but fails with weird behavior in corner cases, I'd like to officially break my program for python 2 with a clear "please use python 3 instead" error message. What's the most reasonable way to fail with errors for unsupported python versions?</p>
  
  <p>For instance, I'd like importing the code in python 2 (at least the top-level package) to trigger an error and for my distutils setup.py script to error out when attempting to install or build it for python 2 (even when invoked via python 3 like <code>python3 setup.py sdist_dsc --with-python2</code>). I'd also like to include any relevant metadata to officially declare which python versions I support. There's a Requires-Python field in <a href="http://legacy.python.org/dev/peps/pep-0345/" rel="noreferrer">PEP 345</a> but I don't think that matters if I'm not using distutils2.</p>