Expose the CO_xxx flags via the "new" module (re-solving a problem "the

Expose the CO_xxx flags via the "new" module (re-solving a problem "the
right way").  Fiddle __future__.py to use them.

Jeremy's pyassem.py may also want to use them (by-hand duplication of
magic numbers is brittle), but leaving that to his judgment.

Beef up __future__'s test to verify the exported feature names appear
correct.