some flake8 cleanup in manifolds

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

mainly about E701 and E702 : one command per line

CC:  @egourgoulhon

Component: **manifolds**

Author: **Frédéric Chapoton**

Branch/Commit: **[`dc768e4`](https://github.com/sagemath/sagetrac-mirror/commit/dc768e4bf97419b24b019a942448dc5c317532a2)**

Reviewer: **Eric Gourgoulhon**

_Issue created by migration from https://trac.sagemath.org/ticket/30538_

