Fix Sphinx 5 warning by setting docs language

[hugovk]

New in Sphinx 5:

#10062: Change the default language to 'en' if any language is not set in conf.py

https://www.sphinx-doc.org/en/master/changes.html

Before

Running Sphinx v5.0.1
WARNING: Invalid configuration value found: 'language = None'. Update your configuration to a valid langauge code. Falling back to 'en' (English).
making output directory... done
...

https://readthedocs.org/projects/pillow/builds/17091425/

After

Running Sphinx v5.0.1
making output directory... done
...

https://readthedocs.org/projects/pillow-hugovk/builds/17091435/