fix E251 in schemes

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about

```
E251 unexpected spaces around keyword / parameter equals
```

Component: **algebraic geometry**

Author: **Frédéric Chapoton**

Branch/Commit: **[`a7949bf`](https://github.com/sagemath/sagetrac-mirror/commit/a7949bfc062a14de28f36fe9bc162ce2fd5d92e4)**

Reviewer: **David Coudert**

_Issue created by migration from https://trac.sagemath.org/ticket/34320_

