py3: remove some Python 2 crufts
covers_lastfm: Fetch covers from Last.fm

The API key should be changed to something specific to Sonata.

It lacks currently any errors checking. It should at least do something if on
of the error code from http://www.last.fm/api/show?service=290 is returned.
Also, it seems sometimes Last.fm returns broken XML, so we should do
something about it.
covers_lastfm: Handle multiple images and errors checking
Port to Python3
covers-lastfm: make the plugin executable by finding some test cover
covers lastfm: new API key