Allow copying algebraic closure of finite field

[user202729]

Fixes https://github.com/sagemath/sage/issues/38988

I choose to make the copy return itself, because that's what GF(p) does at the moment anyway, and the field is immutable.

Not sure how much of a hack for me to implement __copy__ and __deepcopy__ instead of __reduce__, but pickling and unpickling at the moment works well anyway (although it returns a different instance)

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• [x] The description explains in detail what this PR is about.
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• [x] I have created tests covering the changes.
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:hourglass: Dependencies

[github-actions[bot]]

Documentation preview for this PR (built with commit f7e45486319469418089eed912283901a706ebdc; changes) is ready! :tada: This preview will update shortly after each push to this PR.