autodoc: autodoc_type_aliases does not effect to the type annotation for instance attributes

[tk0miya]

Describe the bug
autodoc: autodoc_type_aliases does not effect to the type annotation for instance attributes

To Reproduce

# example.py
from __future__ import annotations

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

# index.rst

.. autoattribute:: example.Foo.attr

# conf.py
autodoc_type_aliases = {'myint': 'mod.myint'}

Expected behavior
The type annotations for the instance variables also changed by autodoc_type_aliases

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