Decide what to do with the PyPI ets package

There's an `ets` package on PyPI, but it's uninstallable with Python 3: `pip install ets` fails on a use of `execfile` in the setup script.

Deleting packages from PyPI isn't something that should be done lightly - someone somewhere might have a workflow that still depends on this package. Instead I'd suggest that we update the README and make a bugfix "release", so that the PyPI description indicates clearly that the package is no longer supported.