Fix byte-compilation with Emacs 27.

--- treepy.el-0.1.1-orig/treepy.el
+++ treepy.el-0.1.1/treepy.el
@@ -40,6 +40,7 @@
 ;; 
 ;;; Code:
 
+(require 'cl-lib)
 (require 'map)
 
 ;;; Walk (recursive tree traversal)