gh-36922: using less isinstance(..., Ring)
in favour of the more categorical `... in Rings()`
also fixing a few pep8 details in passing but not too much
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URL: https://github.com/sagemath/sage/pull/36922
Reported by: Frédéric Chapoton
Reviewer(s): Frédéric Chapoton, Matthias Köppe, Travis Scrimshaw
