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.
The parens (and comma) around self.conn... are unnecessary (unless that changed in python 3?).