The ones I can think off:
  - Test `len(stack)` for the `complex_stack` case.
  - Test contains as in `sentinel.k in stack` for the complex case.
  - Test `stack.keys()` and `stack.values`
  - Push a `DictionaryStack` inside another :grin:, pop something from the original one?

The first three cases above are done by collections.Mapping, however it will be nice to have tests if we ever change implementation.