Change some instances of "gens" method to return tuples

[pyrusbrawler64]

Contributes to #34120

As noted in the referenced issue. most instances of the "gens" method return tuples, so the few instances of the method which return lists instead should be modified to return tuples. This pull request handles the following bullet points from the issue:

• SymmetricReductionStrategy in src/sage/rings/polynomial/symmetric_reduction.pyx
• EllipticCurve_number_field in src/sage/schemes/elliptic_curves/ell_number_field.py
• SpecialCubicQuotientRing in src/sage/schemes/hyperelliptic_curves/monsky_washnitzer.py

In each case the gens method from the mentioned class has been modified to return a tuple instead of a list and doctests have been updated appropriately.

📝 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

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[vincentmacri]

LGTM