fix: sphinx "invalid language code"

Problem:

    docs: commands[0] …/docs> sphinx-build -b html -d …/.tox/docs/tmp/doctrees . …/py
    Running Sphinx v6.2.1
    WARNING: Invalid configuration value found: 'language = None'.
    Update your configuration to a valid language code. Falling back to 'en' (English).

Solution:

    Set language.

docs/conf.py

@@ -68,7 +68,7 @@
 #
 # 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
+language = 'en'
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.