This was a vestige from when we used to need the unittest2 library for pre-2.7
compatibility. Now that we require Python 2.7, we aren't using that library
and this indirection wasn't doing any good.
* I don't see DefaultList be really helpful in many other cases, so having
a beets.util.collections module (which could also conflict with the collections
module from standard library) with only that in it is a little silly.
* It was elegant and concise, but there are implementation issues: it is not
recommended to subclass builtin types, but the alternatives differ between python
2 and 3 (subclass sequence or container or collections.abc?), moreover,
interpreters can differ in the way they map syntaxic sugar to magic functions.
* Also, slicing and negative indexing could do weird things, so the class wouldn't
be really intuitive and simple at the same time