Permit _AttrGetter to override absent docstrings from __slots__

[clayote]

Purpose

Last year, attrs changed its implementation of @cached_property on classes with slots in a way that made those properties invisible to Sphinx's autodoc. It now writes a custom __getattr__ that accesses the slot by the same name as the property.

Sphinx has an easy solution: the add_autodoc_attrgetter hook lets us swap in the original cached_property for documentation purposes. This works fine for the .. autoproperty directive. However, I also wanted the :members: option of .. autoclass to pick up any @cached_property decorated in the class definition. This did not work, because the member-finder code didn't anticipate the necessity. Once it finds the slots member, it considers the member found, even if there's no docstring for it.

This PR lets the general member-finding code for classes override an earlier result, in the case that the prior ObjectMember has no docstring. The special attrgetter code gets a chance to run, and delivers the object we really want to document.

References

• attrs issue #1325

[clayote]

Here is the extension to make @cached_property work with sphinx-ext-autodoc and attrs. It requires the change in this PR.