Fix docs build for newer Sphinx

The language configuration option must now be set, and will issue a
warning when unset.
https://www.sphinx-doc.org/en/master/changes.html#id15

docs/conf.py

@@ -78,7 +78,7 @@ def chdir(directory):
 #
 # 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.