gh-37023: Parent in plural
Using `Parent` and `CommutativeRing` in the plural file
This is a little move towards getting rid of the old coercion framework.
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
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URL: https://github.com/sagemath/sage/pull/37023
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe
