DOC: Set language in Sphinx config to en

Starting with Sphinx version 5.0, the configuration for the doc's
language should not be None anymore and causes docbuild failures
otherwise (see datalad/datalad#6715). This change sets the language
specification to English (en)

docs/source/conf.py

@@ -101,7 +101,7 @@
 #
 # 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.