Refactor period lattice

[user202729]

Improvement for #38474 . (previously I didn't know is_exact() exists, thus the ugly isinstance() check.)

Also now it works with RDF and CDF.

📝 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

Requires #39137 .

[github-actions]

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[kwankyu]

Check the documentation as well if you already didn't.

[user202729]

On another note, apparently PeriodLattice is also a Parent so it has its own is_exact(), but I decide to not touch this.

There are three ways this could be feasibly implemented

• always return False — this is because .basis() always return elements in inexact fields;
• return .curve().is_exact() — (no particularly good reason)
• throw an error — after all this is more like a facade parent (you can't construct an element with parent() equal to the period lattice)

but that should not in the scope of this PR.

[kwankyu]

Thanks. LGTM.

[user202729]

Thanks. (the code introduced by this was one of my first pull requests, and evidently my coding style was not the best back then)