Bug in Poset.is_slender(?)

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CC:  @mezzarobba @jm58660

Component: **combinatorics**

Author: **Marc Mezzarobba, Jori Mäntysalo, Frédéric Chapoton**

Branch/Commit: **[`7608202`](https://github.com/sagemath/sagetrac-mirror/commit/7608202b228c7d6ff5388d19632f415a575f13b1)**

Reviewer: **Jori Mäntysalo, Frédéric Chapoton**

_Issue created by migration from https://trac.sagemath.org/ticket/22373_

