py3 remove __cmp__ from Element

<div id="comment:0"></div>

as another step to py3

CC:  @tscrim @jdemeyer @a-andre @jhpalmieri

Component: **python3**

Author: **Frédéric Chapoton**

Branch/Commit: **[`6124122`](https://github.com/sagemath/sagetrac-mirror/commit/61241224f0b95c91e6000108669de70cb2e827c6)**

Reviewer: **Jeroen Demeyer**

_Issue created by migration from https://trac.sagemath.org/ticket/22297_

