SortedList: Don't inherit from UserList.

A class should only inherit from UserList if the type of data it presents is
the same as the data in the 'data' attribute, which isn't true here.

This means, for example, that SortedList.contains checked if the value
is in self.data, which always returns False (unless unlucky, but then it
returns True while it shouldn't).

By removing this inheritance, methods that are no longer implemented no longer
default to a buggy implementation.