Inherit pandas classes from abc.collections classes?
What do people think about inheriting pandas classes from `abc.collections` classes? 

I think the main ones are `DataFrame` & `Series` inheriting from `Mapping` and `Index` from `Sequence`.

This would enable other libraries to infer classes' behavior in a tighter way, for example to work out whether an object is list-like but not dict-like, they can check the types, rather than `hasattr(obj, '__iter__') and not hasattr(obj, 'values')`. And it doesn't cost anything.

But as far as I'm aware, other libraries in the pydata ecosystem don't do this, so this would be out of the norm.

https://docs.python.org/3/library/collections.abc.html
