autoattribute could not create document for __slots__ attributes correctly

[tk0miya]

Describe the bug
autoattribute could not create document for slots attributes correctly

To Reproduce

# example.py
class Foo:
    __slots__ = {'attr': 'docstring'}

# index.rst
.. autoattribute:: example.Foo.attr

The build succeeded. But docstring is not shown.

Expected behavior
Document should be generated successfully like when I used automodule.

Your project
N/A

Screenshots
N/A

Environment info

• OS: Mac
• Python version: 3.9.0
• Sphinx version: HEAD of 3.x
• Sphinx extensions: sphinx.ext.autodoc
• Extra tools: Nothing

Additional context
Nothing