autodoc: autoattribute directive can't create document for commented attribute in alias class

[tk0miya]

Describe the bug
autodoc: autoattribute directive can't create document for commented attribute in alias class

To Reproduce

# example.py

class Foo:
    def __init__(self):
        self.attr = None  #: docstring

Alias = Foo

# index.rst

.. autoattribute:: example.Alias.attr

This example causes ImportError on generating a document.

Expected behavior
Build successfully.

Your project
No

Screenshots
No

Environment info

• OS: Mac
• Python version: 3.9.1
• Sphinx version: HEAD of 3.x
• Sphinx extensions: sphinx.ext.autodoc
• Extra tools: No

Additional context
No