gh-37707: Implement an iterator for absolute number fields
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We use the $\mathbb{Q}$ vector space structure. Although because of
#37706, we go through `cartesian_product`.
If there is an easy way to implement this in full generality (i.e., all
number fields), please let me know.
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URL: https://github.com/sagemath/sage/pull/37707
Reported by: Travis Scrimshaw
Reviewer(s): Matthias Köppe
