Fix neovim mode py3 compatibility issues

Fixes an attempt in neovim_mod.py to decode what in python3 was already a decoded string. This is done by configuring neovim to always return decoded strings and then using a `u'%s' % (…,)` trick to coerce to unicode in py2 and str in py3.
They're intentional. I'm being a little paranoid, but if e.g. eval returns a dictionary, thee behavior will be wrong without this explicit tuple syntax.