autosummary doesn't document annotated attributes with no default values

Describe the bug

For the following code, A.a won't be documented by autosummary:

class A:
    "A's doc"

    a: int
    "Doc"

To Reproduce
Steps to reproduce the behavior:

1. Download bug.tar.gz and unarchive it.

2. Run PYTHONPATH=. sphinx-build -d "./_build" docs "_build/html" -b html -W --color

.                                                                                                                                                                                             
├── docs                                                                                                                                                                                      
│   ├── conf.py
│   └── index.rst
└── myproject.py

myproject.py defines class A as above. index.rst shows:

Welcome to myproject's documentation!
=====================================

.. currentmodule:: myproject

.. autosummary::
   :toctree:

   A

conf.py is minimal:

project = 'myproject'
copyright = '2020, someone'
author = 'someone'
extensions = [
    'sphinx.ext.autosummary'
]
html_theme = 'alabaster'
autosummary_generate = True

Expected behavior
A.a being documented.

Environment info

• OS: Debian 10
• Python version: 3.7.3
• Sphinx version: 3.4.1
• Sphinx extensions: autosummary

Start by downloading and unpacking bug.tar.gz, then run the reported sphinx-build command against docs with warnings treated as errors. Trace how the autosummary entry for A is generated from myproject.py, and verify completion when the generated documentation includes A.a for the annotated attribute without a default value.