Add module specification: itemgetter -> operator.itemgetter (GH-12823)

Doc/library/itertools.rst

@@ -827,7 +827,7 @@ which incur interpreter overhead.
        "List unique elements, preserving order. Remember only the element just seen."
        # unique_justseen('AAAABBBCCDAABBB') --> A B C D A B
        # unique_justseen('ABBCcAD', str.lower) --> A B C A D
-       return map(next, map(itemgetter(1), groupby(iterable, key)))
+       return map(next, map(operator.itemgetter(1), groupby(iterable, key)))
 
    def iter_except(func, exception, first=None):
        """ Call a function repeatedly until an exception is raised.