Revert "add_stylesheet is deprecated. Use add_css_file instead"

Since we still use older sphinx we need to live with the deprecation warnings until we can figure out a way to support old and new.

This reverts commit 84d9d363c9.

docs/conf.py

@@ -16,7 +16,7 @@ import sys
 import os
 
 def setup(app):
-	app.add_css_file('zmstyle.css')
+	app.add_stylesheet('zmstyle.css')
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the