...

It's also worth remembering that there will be implementations other
than CPython that will need changes, too - Jython, PyPy, possibly
Cython, and many editors and IDEs. So setting the bar at "someone who
wants this will have to step up and provide a patch" seems reasonable
to me.

It would be more or less trivial for Jython to add such support, given that Java has such support natively, and we already leverage this support in our current implementation of 2.7. See https://docs.oracle.com/javase/7/docs/api/java/lang/Double.html#valueOf(java.lang.String) if curious. We just need to add the correct floating point constant that is parsed to Java's constant pool, as used by Java bytecode, and it's done.

I'm much more concerned about finishing the rest of 3.x.

- Jim