bpo-38336: Remove the __set__ method restriction on data descriptors for attribute lookup precedence

[geryogam]

The language documentation states:

Data descriptors with __set__() and __get__() defined always override a redefinition in an instance dictionary. In contrast, non-data descriptors can be overridden by instances.

This override is not limited to data descriptors defining __set__() (and __get__()). It is also true for data descriptors defining __delete__() (and __get__()) and data descriptors defining both __set__() and __delete__() (and __get__()).

In other words, any data descriptors (objects defining either __set__() or __delete__() or both) defining __get__() override an attribute redefinition in an instance dictionary.

https://bugs.python.org/issue38336

[brandtbucher]

Thanks @maggyero, looks good to me!

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[geryogam]

I have made the requested changes; please review again.

[bedevere-bot]

Thanks for making the requested changes!

@rhettinger: please review the changes made to this pull request.

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[geryogam]

I have made the requested changes; please review again.

[bedevere-bot]

Thanks for making the requested changes!

@rhettinger: please review the changes made to this pull request.

[geryogam]

Thanks @rhettinger for reviewing this.