Finish changing Rational's round method default rounding to even

[user202729]

Follow-up to #35756 .

(As is, the warning cannot be suppressed)

Related: #37306, #21935

There's a little (large??) problem: previously K2.<sqrt2> = QuadraticField(2); K2(9/2).round() returns 5 without a warning (because #35756 forget to print the deprecation). Should we add the deprecation first then change this next year?

📝 Checklist

• The title is concise and informative.
• 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.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit d8dfdcb; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[vneiger]

In fact, I would say the deprecation acts as intended, cf DeprecationWarning :
Base category for warnings about deprecated features when those warnings are intended for other Python developers (ignored by default, unless triggered by code in main).

Therefore, since it has been about 18 months since the deprecation introduced in #35756, it could indeed be the right time to actually make the change (from default away to even).

[vneiger]

It seems to me that the failing doctests are not linked to the changes here (one I cannot reproduce, and the other one I can reproduce but already without the changes of this PR). The changes look good to me.

[vneiger]

Another set of eyes may be useful before a positive review. Maybe @mezzarobba who had also contributed to #35756 ?