graph: Rename concat() to flatten_list()

pisi/graph.py

@@ -146,7 +146,7 @@ class Digraph(object):
                 return '_'
             else:
                 return char
-        return pisi.util.concat(map(repl, str(u)))
+        return pisi.util.flatten_list(map(repl, str(u)))
 
     def write_graphviz(self, f):
         f.write('digraph G {\n')