gh-36628: some simplifications in doctest/ folder (ruff C4)
use `ruff check --fix --select=C4 src/sage/doctest` to fix a few details
plus some manual tweaking afterwards
### :memo: Checklist
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URL: https://github.com/sagemath/sage/pull/36628
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe
