Remove obscure mixin parameter from Storage.open() method
|Reported by:||Jannis Leidel||Owned by:||nobody|
|Has patch:||no||Needs documentation:||no|
|Needs tests:||no||Patch needs improvement:||no|
As discussed with Jacob, Russell and Marty this ought to be removed since it's an undocumented and rather obscure feature which can easily be implemented with standard subclassing/overriding.