Update for Sphinx 5.3 based on warnings

$ sphinx-build -b html . _build/html
Running Sphinx v5.3.0
WARNING: Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English).
WARNING: The canonical_url option is deprecated, use the html_baseurl option from Sphinx instead.

conf.py

@@ -72,7 +72,7 @@ release = u''
 #
 # 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.
@@ -101,7 +101,7 @@ html_css_files = [
 # TIP: If it starts with `html_` it belongs directly as a conf.py option and
 # NOT in html_theme_options.
 html_theme_options = {
-    'canonical_url': 'https://docs.agaric.coop/',
+    'html_baseurl': 'https://docs.agaric.coop/',
     'logo_only': True,
 }