gh-100980: ctypes: Test, document, and fix finalizing _fields_

[encukou]

• If setting _fields_ fails, e.g. with AttributeError, don't set the attribute in __dict__
• Document the “finalization” behaviour
• Beef up tests: add getattr, test Union as well as Structure
• Put common functionality in a common function

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P.S. del X._fields_ is still allowed; preventing that is out of scope of this change. Like mutating the list, it doesn't have an effect. (_fields_ is read once, when the class is finalized, and not consulted again.)

• Issue: ctypes Stucture._fields_ cannot be set when Structure has been subclassed #100980

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📚 Documentation preview 📚: https://cpython-previews--124292.org.readthedocs.build/