Remove unused 'attic' module

We don't actually import this anywhere, and most of the functions in it are trivial - we even have a `popkey` function, although dictionaries have had a built-in `pop()` method since Python 2.3.

The only non-trivial bit is `map_method` function, which lets you do things like `map_method('upper', list_of_strings)`. But we're still not using it anywhere, and I suspect we'd now use a listcomp in most cases instead of that.