thumbnails: Python 3 compatibility

### Problem

When using the thumbnail plugin with python 3 I get the following error.

Running this command in verbose (`-vv`) mode:

```sh
$beet -vv thumbnails genre:Classical
```

Led to this problem:

```
(.env_beets) pierre@pierre-System-Product-Name:~$ beet -vv thumbnails genre:Classical
user configuration: /home/pierre/.config/beets/config.yaml
data directory: /home/pierre/.config/beets
plugin paths:
Sending event: pluginload
artresizer: method is (2, (6, 8, 9))
thumbnails: using IM to write metadata
thumbnails: using GIO to compute URIs
library database: /home/pierre/.config/beets/music-collection.db
library directory: /home/pierre/DataDrive/MusicTri
Sending event: library_opened
thumbnails: using IM to write metadata
thumbnails: using GIO to compute URIs
thumbnails: generating thumbnail for Johann Sebastian Bach - Bach 2000: The Complete Bach Edition, Volume 2: Sacred Cantatas: BWV 48-52, 54-69, 69A, 70-99 Classical - 1999 - /home/pierre/DataDrive/MusicTri/Classical/Bach, Johann Sebastian/Bach 2000_ The Complete Bach Edition, Volume 2_ Sacred Cantatas_ BWV 48-52, 54-69, 69A, 70-99 - 1999 - Johann Sebastian Bach
Traceback (most recent call last):
File "/home/pierre/.env_beets/bin/beet", line 9, in
load_entry_point('beets==1.4.2', 'console_scripts', 'beet')()
File "/home/pierre/.env_beets/lib/python3.5/site-packages/beets/ui/__init__.py", line 1209, in main
_raw_main(args)
File "/home/pierre/.env_beets/lib/python3.5/site-packages/beets/ui/__init__.py", line 1196, in _raw_main
subcommand.func(lib, suboptions, subargs)
File "/home/pierre/.env_beets/lib/python3.5/site-packages/beetsplug/thumbnails.py", line 78, in process_query
self.process_album(album)
File "/home/pierre/.env_beets/lib/python3.5/site-packages/beetsplug/thumbnails.py", line 132, in process_album
wrote &= self.make_cover_thumbnail(album, 256, LARGE_DIR)
File "/home/pierre/.env_beets/lib/python3.5/site-packages/beetsplug/thumbnails.py", line 144, in make_cover_thumbnail
target = os.path.join(target_dir, self.thumbnail_file_name(album.path))
File "/home/pierre/.env_beets/lib/python3.5/site-packages/beetsplug/thumbnails.py", line 166, in thumbnail_file_name
hash = md5(uri.encode('utf-8')).hexdigest()
AttributeError: 'bytes' object has no attribute 'encode'
```

### Setup

* OS: linux mint rebecca
* Python version: 3.5.2
* beets version: 1.4.2
* Turning off plugins made problem go away (yes/no): yes, the problem only happens when activating the thumbnail plugin