py3: some fix in set_partition

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



CC:  @tscrim

Component: **python3**

Author: **Frédéric Chapoton, Martin Rubey**

Branch/Commit: **[`171f988`](https://github.com/sagemath/sagetrac-mirror/commit/171f9885ff150378e69de4cdfeea2fb1d6b107ea)**

Reviewer: **Frédéric Chapoton, Travis Scrimshaw**

_Issue created by migration from https://trac.sagemath.org/ticket/26917_

