Make RingExtension work with tower of finite field extension

[user202729]

This pull request makes RingExtension work over nontrivial tower of extension constructed using PolynomialQuotientRing. See the tests for details (previously they fail).

            sage: p = 886368969471450739924935101400677
            sage: K = GF(p)
            sage: Kx.<x> = K[]
            sage: K3.<u> = K.extension(Kx([4, 1, 0, 1]))
            sage: K3y.<y> = K3[]
            sage: K6.<t> = K3.extension(K3y([2, 0, 1]))
            sage: K6t.<t1> = K6.over(K, gen=t)
            Traceback (most recent call last):
            ...
            ValueError: the given family is not a basis
            sage: K6t.<t1> = K6.over(K, gen=t+u)
            sage: K6t(t1).minpoly()
            x^6 + 8*x^4 + 8*x^3 + 13*x^2 + 886368969471450739924935101400637*x + 18
            sage: K6t(t1).minpoly()(t1)
            0

The example is taken from #40764. For now, you need to provide a generator t+u.

📝 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 d096221; changes) is ready! 🎉
This preview will update shortly after each push to this PR.