using items in indexed_elements.pyx

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instead of the old-style "iteritems"

CC:  @tscrim

Component: **refactoring**

Author: **Frédéric Chapoton**

Branch/Commit: **[`4676b51`](https://github.com/sagemath/sagetrac-mirror/commit/4676b51a0f360a33b0b1bdddf644d6f499f18e64)**

Reviewer: **Travis Scrimshaw**

_Issue created by migration from https://trac.sagemath.org/ticket/34505_

