jreese reminded me of pep570, which will make more grammar changes.  I'm open to the idea of replacing the grammar with the live one, plus porting the 2isms forward like print, eval, except with comma.

My sincere hope is that everyone that depends on this structure will have tests (mine and lib2to3 do); the only big user I'm aware of is probably libfuturize.  Definitely worth a changelog entry if this is the way forward.