PyNumber protocol does not check for implementations in base classes

[arihant2math]

PySequence and others do this, but PyNumber does not. This leads to #5711 and ctypes data multiplication not working.

I haven't checked if cpython just expects slots to be inherited or does this.

[arihant2math]

@youknowone I'm curious as to how I could attempt a fix for this. It seems as though PyNumber has been defined slightly differently than the other protocols, and I think this is intentional.

[youknowone]

It's been so long time ago I worked on slots. I forgot the most of details.

CPython basically inherits slots not to require subtypes to crawl up the entire mro. But we don't have it. We check base classes to avoid this limitation. About PyNumber, checking base classes made another problem like referencing PyBaseObject's PyNumber, which shouldn't happen.

If PyNumber slots are fairly well isolated from others, we will be able to implement slot inheritance only for PyNumber