MCVE to reproduce sphinx's classvar appending

docs.conf.py

import sys

sys.path.append('src')

project = "sphinx_test"

extensions = [
    "sphinx.ext.autodoc",
    "sphinx.ext.napoleon",
]

docs.index.rst

Doc Index

.. toctree::
   :maxdepth: 3
   :glob:

   autodoc/*

src.dc.py

import dataclasses

@dataclasses.dataclass
class Foo:
    """Docstring for Foo"""
    var_a: str
    var_b: int

store files as:

.
├── docs
│   ├── conf.py
│   └── index.rst
└── src
    └── dc.py

Then build documentation and open documentation by running

$ python3.7 -m venv venv  # or 3.8, or 3.9
$ source venv/bin/activate
(venv) $ pip install sphinx
(venv) $ sphinx-apidoc --no-toc -o docs/autodoc src/
(venv) $ sphinx-build -b html docs/ public
(venv) $ xdg-open public/autodoc/dc.html

The offending items are the var_a and var_b parts at the bottom of the class description that weren't part of the class docstring: