autodoc: The annotation only member in superclass is treated as "undocumented"

[tk0miya]

Describe the bug
autodoc: The annotation only member in superclass is treated as "undocumented".

To Reproduce

# example.py
class Foo:
    """docstring"""
    attr1: int  #: docstring


class Bar(Foo):
    """docstring"""
    attr2: str  #: docstring

# index.rst
.. autoclass:: example.Bar
   :members:
   :inherited-members:

Bar.attr1 is not documented. It will be shown if I give :undoc-members: option to the autoclass directive call. It seems the attribute is treated as undocumented.

Expected behavior
It should be shown.

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