EncodingWarning

[jaraco]

When building docs with furo enabled and PYTHONWARNDEFAULTENCODING=1, I see this warning:

/Users/jaraco/code/python/importlib_resources/.tox/docs/lib/python3.11/site-packages/furo/__init__.py:387: EncodingWarning: 'encoding' argument not specified
  with open(os.path.join(app.builder.outdir, "_static", "pygments.css"), "w") as f: