doc build fails with Sphinx 5.0

Kea fails to build documentation with Sphinx 5.0 (currently in Debian experimental). Relevant excerpt:

make[4]: Entering directory '/<<PKGBUILDDIR>>/doc/sphinx'
rm -f ./arm/platforms.rst
cp ./../../platforms.rst ./arm/platforms.rst
/usr/bin/sphinx-build -M html . ./_build -v -E -a -W -j 2 -c "/<<PKGBUILDDIR>>/doc/sphinx"
Running Sphinx v5.0.2

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/sphinx/cmd/build.py", line 272, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
  File "/usr/lib/python3/dist-packages/sphinx/application.py", line 196, in __init__
    self.config = Config.read(self.confdir, confoverrides or {}, self.tags)
  File "/usr/lib/python3/dist-packages/sphinx/config.py", line 172, in read
    logger.warning(__("Invalid configuration value found: 'language = None'. "
  File "/usr/lib/python3.10/logging/__init__.py", line 1847, in warning
    self.log(WARNING, msg, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/sphinx/util/logging.py", line 122, in log
    super().log(level, msg, *args, **kwargs)
  File "/usr/lib/python3.10/logging/__init__.py", line 1879, in log
    self.logger.log(level, msg, *args, **kwargs)
  File "/usr/lib/python3.10/logging/__init__.py", line 1547, in log
    self._log(level, msg, args, **kwargs)
  File "/usr/lib/python3.10/logging/__init__.py", line 1624, in _log
    self.handle(record)
  File "/usr/lib/python3.10/logging/__init__.py", line 1634, in handle
    self.callHandlers(record)
  File "/usr/lib/python3.10/logging/__init__.py", line 1696, in callHandlers
    hdlr.handle(record)
  File "/usr/lib/python3.10/logging/__init__.py", line 964, in handle
    rv = self.filter(record)
  File "/usr/lib/python3.10/logging/__init__.py", line 821, in filter
    result = f.filter(record)
  File "/usr/lib/python3/dist-packages/sphinx/util/logging.py", line 425, in filter
    raise exc
sphinx.errors.SphinxWarning: Invalid configuration value found: 'language = None'. Update your configuration to a valid langauge code. Falling back to 'en' (English).

Warning, treated as error:
Invalid configuration value found: 'language = None'. Update your configuration to a valid langauge code. Falling back to 'en' (English).
make[4]: *** [Makefile:902: html] Error 2

This can be fixed by replacing language = None with language = 'en' in conf.py.