gh-38741: move power series to Parent
make the power series rings inherit from Parent instead of the auld
`Ring` class
### :memo: Checklist
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.
URL: https://github.com/sagemath/sage/pull/38741
Reported by: Frédéric Chapoton
Reviewer(s): Frédéric Chapoton, Martin Rubey
