[PATCH evolve-ext] py3: broad pass for python3 compatibility

[Ludovic Chabant]

> Does this make any existing tests for evolve pass on py3?

The evolve extension wasn't even loading in py3 before this patch --
mercurial core would reject it for trying to add unicode strings to the
command table.

> Taking other patches first would mean this patch would be hard to
> apply and it would miss any newly added code, so the question is: how
> hard would it be to create this patch again?

It's easy to make this patch again:

1) run mercurial's contrib/bytify_strings.py script with the --
   dictiter argument

2) fix up the few calls to exthelper.addattr to _not_ use byte strings
   (because those strings are passed to python's addattr() builtin, and
   that wants unicode).

That's it!