Fix docs build

> Warning, treated as error:
>   Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English).

docs/conf.py

@@ -61,7 +61,7 @@ master_doc = 'index'
 #
 # 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.