py3: fix manifolds again

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

after a regression

CC:  @egourgoulhon

Component: **python3**

Author: **Frédéric Chapoton**

Branch/Commit: **[`966209a`](https://github.com/sagemath/sagetrac-mirror/commit/966209a5ebf9e95ea430268ab3913792a799bee0)**

Reviewer: **Travis Scrimshaw**

_Issue created by migration from https://trac.sagemath.org/ticket/26569_

