fix and activate W605 and E306 in pyx files

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Component: **refactoring**

Author: **Frédéric Chapoton**

Branch/Commit: **[`fda9085`](https://github.com/sagemath/sagetrac-mirror/commit/fda90859c17b01922d18086b64ad86bbca8f6948)**

Reviewer: **Matthias Koeppe**

_Issue created by migration from https://trac.sagemath.org/ticket/34705_

