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/util/hidden.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # This file is part of beets.
 # Copyright 2016, Adrian Sampson.
 #
@@ -14,7 +13,6 @@
 # included in all copies or substantial portions of the Software.
 
 """Simple library to work out if a file is hidden on different platforms."""
-from __future__ import division, absolute_import, print_function
 
 import os
 import stat