Here's an attempt at implementing this (my first contribution). Some notes:

- I tried to keep `__repr__()` backwards compatible. It would have been easier to inherit from `_AttributeHolder`, but maybe this might break some things...
- I added some test cases for `__repr__()`, but that's it. I don't think there's any other sensible test to do, as we're really just passing stuff to `open()`
- not sure about the style, especially line breaks...