[wladimir]

Yes, a backward compatibility mode that could re-add those things would be nice (which could be enabled by adding a pragma "from __past__ import old_unicode_literals", for example). It would allow for a more gradual transition.

[mcdonc]

The pragma wouldn't really work. It'd have to work in all old versions of Python to make any sense, and it doesn't. For unicode literals in particular, it should just be true in Python 3.3+ that u'' = '' out of the box.