Switch sphinx language to en

With sphinx release of 5.0.0, they changed default for language variable
to 'en' from None. With that current None valuable is not valid and should
not be used.


Change-Id: I7d612a82e0b79ad1d34f9cbd75c5e19f201d7741

doc/source/conf.py

@@ -76,7 +76,7 @@ openstackdocs_bug_tag = ''
 #
 # 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'
 
 # There are two options for replacing |today|: either, you set today to some
 # non-false value, then it is used: