Changed `itertools.izip` to `zip`

Python 2 related mistake fixed.

IPython/core/completer.py

@@ -916,7 +916,7 b' class IPCompleter(Completer):'
         # them again
         usedNamedArgs = set()
         par_level = -1
-        for token, next_token in itertools.izip(tokens, tokens[1:]):
+        for token, next_token in zip(tokens, tokens[1:]):
             if token == '(':
                 par_level += 1
             elif token == ')':