Bug report

I am actually not sure if this is a bug. It started happening today for matplotlib installed from your master branch for our CI. We run some image tests using your dev version on CircleCI, which uses our tox.ini and some settings in astropy/conftest.py.

It starts to complain about no backend even though we set it at astropy/conftest.py#L84 and that works for older versions of matplotlib.

INTERNALERROR>   File ".../matplotlib/__init__.py", line 843, in <module>
INTERNALERROR>     fail_on_error=True)
INTERNALERROR>   File ".../matplotlib/__init__.py", line 773, in _rc_params_in_file
INTERNALERROR>     config[key] = val  # try to convert to proper type or raise
INTERNALERROR>   File ".../matplotlib/__init__.py", line 607, in __setitem__
INTERNALERROR>     raise ValueError(f"Key {key}: {ve}") from None
INTERNALERROR> ValueError: Key backend: '' is not a valid value for backend; supported values are
['GTK3Agg', 'GTK3Cairo', 'MacOSX', 'nbAgg', 'Qt4Agg', 'Qt4Cairo', 'Qt5Agg', 'Qt5Cairo',
'TkAgg', 'TkCairo', 'WebAgg', 'WX', 'WXAgg', 'WXCairo', 'agg', 'cairo', 'pdf', 'pgf', 'ps', 'svg', 'template']

Code for reproduction

We can reproduce it on both GitHub Actions and CircleCI using tox + pytest but only with the dev version of matplotlib.

Actual outcome

See traceback above.

Expected outcome

It does not throw ValueError.

Matplotlib version

• Operating system: Ubuntu on Actions, some Linux image on CircleCI
• Matplotlib version (import matplotlib; print(matplotlib.__version__)): matplotlib @ git+https://github.com/matplotlib/matplotlib.git@cc8669fc69b0cc618336a17d324f5fdbf1922f22 (reported by CircleCI)
• Matplotlib backend (print(matplotlib.get_backend())): Agg but somehow not registering (see above)
• Python version: 3.7
• Jupyter version (if applicable): N/A
• Other libraries: There is a list on CircleCI log under pip freeze

Also, the Actions log.

xref astropy/astropy#11458