error in setup.cfg ("unexpected value continuation") if `pytest.ini` is missing

Description:

If running pytest without pytest.ini for following minimal working example, pytest will fail with the error message:

$ pytest
ERROR: /home/fzeiser/met/pytest_error_unexpected_value_continuation/setup.cfg:17: unexpected value continuation

If a pytest.ini is added (remove .bak from the file in the repo), pytest will work as expected. Similarly, if one removes the [options.extras_require] block of setup.cfg, pytest will run as expected.

Overview of files:

├── foo  # package
│   └── __init__.py
├── pytest.ini.bak # remove .bak to get things working 
├── Readme.md
├── setup.cfg # or remove [options.extras_require] block here 
├── setup.py
├── tests
│   └── test_init.py

Environment:

$ pip list
Package       Version
------------- -------
attrs         21.2.0 
iniconfig     1.1.1  
packaging     21.0   
pip           20.0.2 
pkg-resources 0.0.0  
pluggy        0.13.1 
py            1.10.0 
pyparsing     2.4.7  
pytest        6.2.4  # <-- pytest version 
setuptools    44.0.0 
toml          0.10.2 

$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 20.04.2 LTS
Release:	20.04
Codename:	focal

$ python --version
Python 3.8.10

Start with the linked minimal working example, especially setup.cfg, pytest.ini.bak, setup.py, and tests/test_init.py. Run pytest with pytest.ini absent, then with it restored and with [options.extras_require] removed, to isolate the configuration-parsing difference. Done means the minimal example runs without the unexpected value continuation error when pytest.ini is missing.