Hi,

Please add python2-beautifulsoup4 as optional dependency, cause if you enable the lyrics plugin and uses google as source you need this package.

The error when package is missing:

lyrics: To use the google lyrics source, you must provide an API key in the configuration. See the documentation for further details.
lyrics: To use the google lyrics source, you must install the beautifulsoup4 module. See the documentation for further details.
Traceback (most recent call last):
File "/usr/bin/beet", line 9, in <module>
load_entry_point('beets==1.3.18', 'console_scripts', 'beet')()
File "/usr/lib/python2.7/site-packages/beets/ui/__init__.py", line 1250, in main
_raw_main(args)
File "/usr/lib/python2.7/site-packages/beets/ui/__init__.py", line 1236, in _raw_main
subcommands, plugins, lib = _setup(options, lib)
File "/usr/lib/python2.7/site-packages/beets/ui/__init__.py", line 1112, in _setup
plugins = _load_plugins(config)
File "/usr/lib/python2.7/site-packages/beets/ui/__init__.py", line 1098, in _load_plugins
plugins.send("pluginload")
File "/usr/lib/python2.7/site-packages/beets/plugins.py", line 456, in send
for handler in event_handlers()[event]:
File "/usr/lib/python2.7/site-packages/beets/plugins.py", line 439, in event_handlers
for plugin in find_plugins():
File "/usr/lib/python2.7/site-packages/beets/plugins.py", line 287, in find_plugins
_instances[cls] = cls()
File "/usr/lib/python2.7/site-packages/beetsplug/lyrics.py", line 618, in __init__
sources.remove('google')
ValueError: list.remove(x): x not in list

And more doc: http://beets.readthedocs.io/en/v1.3.18/plugins/lyrics.html#activate-google-custom-search

Thanks!