Conflict with Sphinx 5

[UlrichEckhardt]

Describe the bug
Sphinx 5, released end of May, doesn't work, while the previous version does.

To Reproduce
Follow the contributers' guide for setting up. Running the tests fails. To reduce this further, just run the "doc" session:

python3 -m nox --nocolor --session doc
nox > Running session doc
nox > Creating virtual environment (virtualenv) using python3 in .nox/doc
nox > python -m pip install -r doc/requirements.txt docutils
nox > python -m pip install -e .
nox > cd doc
nox > make html O=-W
gcovr -h > examples/gcovr.out
Running Sphinx v5.0.1

Warning, treated as error:
Invalid configuration value found: 'language = None'. Update your configuration to a valid langauge code. Falling back to 'en' (English).
make: *** [Makefile:27: html] Error 2
nox > Command make html O=-W failed with exit code 2
nox > Session doc failed.

Expected behavior

If you just replace the line "sphinx" in doc/requirements.txt with "sphinx < 5", you get this instead:

python3 -m nox --nocolor --session doc
nox > Running session doc
nox > Creating virtual environment (virtualenv) using python3 in .nox/doc
nox > python -m pip install -r doc/requirements.txt docutils
nox > python -m pip install -e .
nox > cd doc
nox > make html O=-W
gcovr -h > examples/gcovr.out
Running Sphinx v4.5.0
loading pickled environment... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 25 source files that are out of date
updating environment: 0 added, 6 changed, 0 removed
reading sources... [100%] license                                                                                                                                                                           
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] output/txt                                                                                                                                                                         
generating indices... genindex done
writing additional pages... search done
copying images... [100%] ../images/screenshot-html-details.example.cpp.png                                                                                                                                  
copying static files... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded.

The HTML pages are in build/html.
nox > cd ..
nox > rst2html5.py --strict README.rst .nox/doc/tmp/README.html
nox > Session doc was successful.

Desktop (please complete the following information):

• OS: Ubuntu 21.10
• GCC version: multiple (8,9,11)
• GCOVR version: 5.1, master

[Spacetown]

I'll check this after my holiday. I prefer to update the configuration and use the newest version of Sphinx.

[UlrichEckhardt]

Fine for me. Thanks for that and enjoy your holiday!

[Spacetown]

Can you create a PR for this? According to sphinx-doc/sphinx#10474 the following block should be removed:

gcovr/doc/source/conf.py

Lines 77 to 83 in ba4de0e

	# The language for content autogenerated by Sphinx. Refer to documentation
	# for a list of supported languages.
	#
	# This is also used if you do content translation via gettext catalogs.
	# Usually you set "language" from the command line for these cases.
	language = None

Don't forget to add yourself to the contributors.