AllowedFilter should return static rather than self

[kosarinin]

Hey, everyone!

Thanks for this fantastic project. We find it very useful in our projects.

In our project, we want to extend the AllowedFilter class, and it seems nothing prevents us from doing that with the only exception - all methods return self and not static.

Please consider merging these changes.

I appreciate your work on this project!

[Nielsvanpach]

Thanks!