 Accept superclasses in NEP-13/17 dispatching (#6710)
 Accept superclasses in NEP-13/17 dispatching (#6710)

* Support __array_function__ dispatching on Array subclasses

https://github.com/dask/dask/pull/6393#discussion_r498536767

* defer to subclasses, but don't hardcode Array

* Accept superclasses in NEP-13/17 dispatching

jakirkham/dask,jakirkham/dask,dask/dask,dask/dask