Target Python version tags

[mahmoud]

There's a fair amount of diversity in the version of Python used by these applications. The survey information alone is interesting, but maybe there's also an opportunity here to direct some of the energies of die-hard py3 converters toward applications looking for help to make the upgrade.

Suggested tag scheme: py2, py3, py23 (for apps that support both), maybe pypy/pypy3/jython if we find any. Testimony from a readme is fine, we don't need to run every app to confirm.