Remove a use of callable() to silence the warning triggered under -3.

Lib/rlcompleter.py

@@ -93,7 +93,7 @@ class Completer:
             return None
 
     def _callable_postfix(self, val, word):
-        if callable(val):
+        if hasattr(val, '__call__'):
             word = word + "("
         return word