gh-36507: Fix implicit noexcept warnings
We have about 7k different occurrences of this warning (about 59k with
multiplicities).
It turns out to be easy to fix automatically, which we do. There's only
one mistake and one doctest error to fix after the automatic step.
We also fix a few warnings in regards of `nogil` should appear at the
end.
Finally, we disable `legacy_implicit_noexcept` option which is no longer
necessary.
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
URL: https://github.com/sagemath/sage/pull/36507
Reported by: Gonzalo Tornaría
Reviewer(s): Matthias Köppe
