We do not have this optional test dependency packaged yet.

--- rdiff-backup-2.2.6/requs/optional.txt.orig
+++ rdiff-backup-2.2.6/requs/optional.txt
@@ -1,7 +1,6 @@
 
 #--- optional ---
 
-pylibacl ; os_name == "posix"
 pyxattr ; platform_system == "Linux"
 # or xattr under SuSE Linux & Co
 psutil