fix and activate W391 in pyx files #34929
Description
W391 blank line at end of file
Component: refactoring
Author: Frédéric Chapoton
Reviewer: Matthias Koeppe
Issue created by migration from https://trac.sagemath.org/ticket/34929
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W391 blank line at end of file
Component: refactoring
Author: Frédéric Chapoton
Reviewer: Matthias Koeppe
Issue created by migration from https://trac.sagemath.org/ticket/34929