Initial python3 changes

Mostly just updating libraries, removing string encoding/decoding,
fixing some edge cases. No new functionality was added in this
commit.

lib/beets/__main__.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # This file is part of beets.
 # Copyright 2017, Adrian Sampson.
 #
@@ -17,7 +16,6 @@
 `python -m beets`.
 """
 
-from __future__ import division, absolute_import, print_function
 
 import sys
 from .ui import main