gh-97588: Move ctypes struct/union layout logic to Python

[encukou]

As discussed in #97702, moving this logic to Python should make it easier to fix the various struct layout bugs in ctypes.

This PR should be strictly a refactoring of the code in main, except the _ctypes.CField class can now be instantiated from Python.

• Issue: ctypes: bit field data does not survive round trip #97588

[picnixz]

First round of comments on the C side. I'll do the Python files now.

[picnixz]

The way you construct the layout depends on whether it's a union or a struct and whether it's gcc or ms layout. I'm not sure whether it's easier to maintain one huge function or if you want to create separate factories. AFACIT, it could be easier for instance to have a function that determines whether it's a gcc layout or not, one function that is responsible for parsing _align_ and one function responsible for the _pack_ attribute. For the rest, they can be kept inside the get_layout function.

For the loop over the input fields, I don't know whether you want to keep a single one or factor it out (it could be clearer to review, but it will likely cause more code so I'm fine with the current implementation).

[encukou]

Thank you for the review!

[encukou]

Thank you for the reviews!
I plan to merge after 3.13 rc2 is out, and then start solving the layout bugs and missing tests :)

[encukou]

RC2 is delayed. I'll merge now, if buildbots pass.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @encukou for commit 96a5c0d 🤖

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[picnixz]

RC2 is delayed. I'll merge now, if buildbots pass.

The "now" being in at least 8 hours 😆