A couple of notes, since this is new to me too. The radiogroup role is in fact valid on a <fieldset>. See the HTML-ARIA spec for details. Additionally, the W3C is no longer publishing their own version of the HTML spec, and are instead working with WHATWG in their repositority.
I recently tested the support of aria-required on the radiogroup role. It has pretty good support, but using the required attribute on each radio button will most likely yield the most robust support.
Thank you for the review @andrewmacpherson
A new patch is attached which addresses all of your concerns.
The last commit had a file permission change that should not have beed added.
I wasn't sure if I should unset the required attributes (required, aria-required, and aria-invalid) from all fieldsets, or just the fieldset for radios. The required attribute is technically invalid on all fieldsets.